specs/002-fullstack-ui-integration/spec.md

# Feature Specification: Full-Stack Integration & UI Experience

**Feature Branch**: `002-fullstack-ui-integration`
**Created**: 2026-01-09
**Status**: Draft
**Input**: User description: "Full-Stack Integration & UI Experience – Phase II Todo Web App"

## User Scenarios & Testing *(mandatory)*

### User Story 1 - Complete Authentication Flow (Priority: P1) 🎯 MVP

A new user visits the application, creates an account, signs in, and immediately sees a clean, responsive interface ready for task management.

**Why this priority**: This is the entry point for all users. Without a seamless authentication experience, users cannot access any functionality. This story validates the entire authentication integration from Spec 2 works end-to-end with proper UI feedback.

**Independent Test**: Navigate to the application URL, complete signup form, verify redirect to signin, sign in with credentials, and land on the task management page with user profile displayed in header.

**Acceptance Scenarios**:

1. **Given** a new user visits the application root URL, **When** they are not authenticated, **Then** they are automatically redirected to the signin page with a link to signup
2. **Given** a user on the signup page, **When** they submit valid credentials (email, password, name), **Then** they see a success message and are redirected to signin page
3. **Given** a user on the signup page, **When** they submit invalid data (weak password, invalid email), **Then** they see clear inline validation errors without page reload
4. **Given** a registered user on the signin page, **When** they submit correct credentials, **Then** they are redirected to the home page with their name displayed in the header
5. **Given** a user on the signin page, **When** they submit incorrect credentials, **Then** they see a generic error message "Invalid email or password" without revealing which field is wrong
6. **Given** an authenticated user on the home page, **When** they click the "Sign Out" button, **Then** their session is cleared and they are redirected to the signin page

---

### User Story 2 - Responsive UI States (Priority: P2)

Users experience appropriate visual feedback during all application states: loading data, empty states, error conditions, and successful operations.

**Why this priority**: Professional applications provide clear feedback. Users should never wonder if the app is working or broken. This story ensures the UI communicates system state effectively.

**Independent Test**: Sign in, observe loading spinner while tasks load, create first task and see empty state disappear, disconnect network and see error state, reconnect and see recovery.

**Acceptance Scenarios**:

1. **Given** a user signs in successfully, **When** the task list is loading, **Then** they see a loading spinner with "Loading tasks..." message
2. **Given** a new user with no tasks, **When** the task list finishes loading, **Then** they see an empty state with "No tasks yet" message and a call-to-action to create their first task
3. **Given** a user viewing their task list, **When** a network error occurs, **Then** they see a clear error message "Unable to load tasks. Please check your connection." with a retry button
4. **Given** a user creating a new task, **When** the API request is in progress, **Then** the submit button shows "Creating..." and is disabled to prevent duplicate submissions
5. **Given** a user on any page, **When** their JWT token expires, **Then** they are automatically redirected to signin with a message "Your session has expired. Please sign in again."
6. **Given** a user completing a task, **When** the update succeeds, **Then** the task UI updates immediately (optimistic update) without requiring a full page reload

---

### User Story 3 - Responsive Design (Priority: P3)

Users can access and use the application seamlessly across desktop, tablet, and mobile devices with appropriate layout adjustments.

**Why this priority**: Modern web applications must work on all screen sizes. This story ensures the UI adapts gracefully to different viewports, making the app accessible to users on any device.

**Independent Test**: Open the application on desktop (1920px), tablet (768px), and mobile (375px) viewports. Verify all functionality is accessible and layouts adjust appropriately.

**Acceptance Scenarios**:

1. **Given** a user on a desktop browser (≥1024px), **When** they view the home page, **Then** they see a three-column layout: task form (left), filters (middle), task list (right)
2. **Given** a user on a tablet (768px-1023px), **When** they view the home page, **Then** they see a two-column layout: task form and filters stacked (left), task list (right)
3. **Given** a user on a mobile device (<768px), **When** they view the home page, **Then** they see a single-column layout with task form, filters, and task list stacked vertically
4. **Given** a user on any device, **When** they interact with buttons and form inputs, **Then** touch targets are at least 44x44px for comfortable interaction
5. **Given** a user on mobile, **When** they view the signin/signup forms, **Then** the forms are centered, readable, and keyboard-friendly with appropriate input types (email, password)
6. **Given** a user on any device, **When** they navigate the application, **Then** all text is readable without horizontal scrolling and maintains proper contrast ratios

---

### User Story 4 - Centralized API Communication (Priority: P4)

All frontend-backend communication flows through a unified API client that handles authentication, error handling, and request/response formatting consistently.

**Why this priority**: Consistent API communication prevents bugs and makes the codebase maintainable. This story ensures all API calls follow the same patterns for auth, errors, and data handling.

**Independent Test**: Review the codebase to verify all API calls use the centralized `fetchAPI` function. Test that JWT tokens are automatically included, 401 errors trigger signin redirect, and error responses are consistently formatted.

**Acceptance Scenarios**:

1. **Given** any component making an API request, **When** the request is initiated, **Then** the JWT token is automatically included in the Authorization header without manual intervention
2. **Given** an API request in progress, **When** the backend returns a 401 Unauthorized, **Then** the user is automatically redirected to signin and their session is cleared
3. **Given** an API request fails, **When** the backend returns an error response, **Then** the error is caught and formatted consistently with `{ detail, error_code, field_errors }` structure
4. **Given** a component making multiple API calls, **When** any call fails, **Then** the error is handled locally without crashing the entire application
5. **Given** the backend is unreachable, **When** an API request times out, **Then** the user sees a clear error message "Unable to connect to server. Please try again later."
6. **Given** a successful API response, **When** the data is returned, **Then** it is automatically parsed as JSON and typed correctly for TypeScript consumers

---

### User Story 5 - Environment Coordination (Priority: P5)

The application runs successfully in local development with proper environment variable configuration and clear setup instructions.

**Why this priority**: Developers and reviewers need to run the application easily. This story ensures environment setup is straightforward and well-documented.

**Independent Test**: Clone the repository, follow README instructions to set up environment variables, start backend and frontend, and verify the application works end-to-end.

**Acceptance Scenarios**:

1. **Given** a developer cloning the repository, **When** they follow the backend README, **Then** they can set up the database, configure environment variables, and start the backend server successfully
2. **Given** a developer with the backend running, **When** they follow the frontend README, **Then** they can configure environment variables and start the frontend development server successfully
3. **Given** both servers running, **When** a user accesses `http://localhost:3000`, **Then** the frontend successfully communicates with the backend at `http://localhost:8000`
4. **Given** environment variables are missing, **When** the application starts, **Then** clear error messages indicate which variables are required (e.g., "BETTER_AUTH_SECRET is required")
5. **Given** the BETTER_AUTH_SECRET differs between frontend and backend, **When** a user tries to sign in, **Then** token verification fails with a clear error message
6. **Given** the database is not running, **When** the backend starts, **Then** it shows a clear error message "Unable to connect to database at [URL]"

---

### Edge Cases

- What happens when a user's JWT token expires while they're actively using the application (e.g., editing a task)?
- How does the system handle rapid successive API calls (e.g., user clicking "Create Task" multiple times)?
- What happens when the backend returns a 500 Internal Server Error?
- How does the UI behave when task titles or descriptions contain special characters, emojis, or very long text?
- What happens when a user tries to access a protected route by manually typing the URL while unauthenticated?
- How does the application handle browser back/forward navigation after signin/signout?
- What happens when localStorage is disabled or unavailable in the browser?
- How does the system handle concurrent edits (user edits same task in two browser tabs)?

## Requirements *(mandatory)*

### Functional Requirements

- **FR-001**: System MUST redirect unauthenticated users from protected routes to the signin page automatically
- **FR-002**: System MUST display user profile information (name or email) in the application header when authenticated
- **FR-003**: System MUST show loading indicators during all asynchronous operations (API calls, page transitions)
- **FR-004**: System MUST display empty states with helpful messages when no data exists (e.g., "No tasks yet. Create your first task!")
- **FR-005**: System MUST show clear error messages when operations fail, with actionable guidance (e.g., "Retry" button)
- **FR-006**: System MUST handle JWT token expiration gracefully by redirecting to signin with an appropriate message
- **FR-007**: System MUST prevent duplicate form submissions by disabling submit buttons during API requests
- **FR-008**: System MUST validate all form inputs on the client side before submission with inline error messages
- **FR-009**: System MUST adapt layout and component sizing based on viewport width (responsive design)
- **FR-010**: System MUST ensure all interactive elements have minimum touch target size of 44x44px for mobile usability
- **FR-011**: System MUST route all API requests through a centralized client that handles authentication automatically
- **FR-012**: System MUST include JWT tokens in Authorization headers for all protected API endpoints
- **FR-013**: System MUST catch and format all API errors consistently across the application
- **FR-014**: System MUST clear user session and redirect to signin on 401 Unauthorized responses
- **FR-015**: System MUST persist authentication state across page refreshes using localStorage
- **FR-016**: System MUST provide clear setup instructions in README files for both frontend and backend
- **FR-017**: System MUST validate that required environment variables are present on application startup
- **FR-018**: System MUST use the same BETTER_AUTH_SECRET in both frontend and backend for JWT verification
- **FR-019**: System MUST display appropriate CORS configuration to allow frontend-backend communication
- **FR-020**: System MUST use Tailwind CSS utility classes exclusively for styling (no inline styles)

### Key Entities

This feature focuses on integration and UI experience rather than introducing new data entities. It leverages existing entities from Specs 1 and 2:

- **User**: Authenticated user with profile information (from Spec 2)
- **Task**: User's todo items with CRUD operations (from Spec 1)
- **AuthSession**: Frontend session state containing JWT token and user profile (from Spec 2)

## Success Criteria *(mandatory)*

### Measurable Outcomes

- **SC-001**: Users can complete the full flow from signup to creating their first task in under 3 minutes
- **SC-002**: All loading states appear within 100ms of initiating an action to provide immediate feedback
- **SC-003**: Empty states provide clear guidance, resulting in 80% of new users creating their first task within 2 minutes
- **SC-004**: Error messages are clear enough that users can resolve issues without external help 90% of the time
- **SC-005**: The application layout adapts correctly to viewport widths from 320px (mobile) to 1920px (desktop) without horizontal scrolling
- **SC-006**: All interactive elements are accessible and usable on touch devices with no accidental clicks
- **SC-007**: JWT token expiration is handled gracefully with zero application crashes or undefined states
- **SC-008**: API errors are caught and displayed consistently across all features with zero unhandled promise rejections
- **SC-009**: Developers can set up and run the application locally in under 10 minutes following README instructions
- **SC-010**: The application works end-to-end in local development with proper environment configuration

## Assumptions *(mandatory)*

1. **Existing Functionality**: Specs 1 (Task CRUD) and 2 (Authentication) are fully implemented and functional
2. **Browser Support**: Modern browsers with ES6+ support (Chrome, Firefox, Safari, Edge - latest 2 versions)
3. **JavaScript Enabled**: Users have JavaScript enabled in their browsers
4. **Network Connectivity**: Users have stable internet connection for API communication
5. **LocalStorage Available**: Browser supports and allows localStorage for session persistence
6. **Development Environment**: Developers have Node.js 18+, Python 3.11+, and PostgreSQL installed
7. **Screen Sizes**: Target devices range from 320px (small mobile) to 1920px (desktop)
8. **Single User Session**: Users are expected to use one browser session at a time (concurrent sessions not optimized)
9. **English Language**: All UI text and error messages are in English
10. **No Offline Support**: Application requires active internet connection (no offline mode)

## Dependencies *(mandatory)*

### Internal Dependencies

- **Spec 1 - Task CRUD**: All task management endpoints must be functional
- **Spec 2 - Authentication & API Security**: JWT authentication and user management must be working
- **Backend API**: FastAPI server must be running and accessible
- **Database**: PostgreSQL database with all migrations applied
- **Environment Variables**: BETTER_AUTH_SECRET, DATABASE_URL, API URLs configured correctly

### External Dependencies

- **Next.js 16+**: Frontend framework with App Router
- **React 18+**: UI library
- **TypeScript 5.x**: Type safety
- **Tailwind CSS 3.x**: Styling framework
- **FastAPI**: Backend framework
- **SQLModel**: ORM for database operations
- **Better Auth**: Authentication library
- **JWT**: Token-based authentication

## Out of Scope *(mandatory)*

The following are explicitly NOT included in this specification:

1. **New Backend Logic**: No new API endpoints or business logic (handled in Spec 1)
2. **New Authentication Mechanisms**: No OAuth, SSO, or MFA (handled in Spec 2)
3. **Advanced Animations**: No complex transitions, animations, or motion design
4. **Design System**: No comprehensive component library or design tokens
5. **Mobile Native Apps**: No iOS or Android native applications
6. **Progressive Web App (PWA)**: No offline support, service workers, or installability
7. **Internationalization (i18n)**: No multi-language support
8. **Accessibility Audit**: No WCAG compliance testing (basic accessibility assumed)
9. **Performance Optimization**: No advanced caching, code splitting beyond Next.js defaults, or CDN setup
10. **CI/CD Pipelines**: No automated testing, deployment, or infrastructure scripts
11. **Docker Deployment**: No production Docker configuration (local development only)
12. **Monitoring & Analytics**: No error tracking, user analytics, or performance monitoring
13. **SEO Optimization**: No meta tags, sitemaps, or search engine optimization
14. **Email Notifications**: No email verification, password reset emails, or notifications
15. **Real-time Features**: No WebSockets, live updates, or collaborative editing

## References

- **@specs/ui/components.md**: Component design specifications (if exists)
- **@specs/ui/pages.md**: Page layout specifications (if exists)
- **@specs/overview.md**: Project overview and architecture (if exists)
- **@specs/architecture.md**: Technical architecture decisions (if exists)
- **Spec 1**: Task CRUD API implementation
- **Spec 2**: Authentication & API Security implementation
- **Next.js App Router Documentation**: https://nextjs.org/docs/app
- **Tailwind CSS Documentation**: https://tailwindcss.com/docs
- **Better Auth Documentation**: https://better-auth.com/docs

## Notes

This specification focuses on polishing and integrating existing functionality rather than building new features. The goal is to create a cohesive, professional user experience that demonstrates the full capabilities of the Phase II Todo Web App for hackathon evaluation.

Key integration points:
- Frontend ↔ Backend: Unified API client with automatic JWT handling
- Authentication ↔ UI: Seamless auth state management across all pages
- Components ↔ Styling: Consistent Tailwind CSS usage throughout
- Development ↔ Production: Clear environment setup and configuration

Success depends on attention to detail in error handling, loading states, and responsive design rather than adding new functionality.