feat: add early username availability check on registration step 1

feat: implement two-step registration form with teacher profile fields

support additional teacher registration information

feat: defer coach selection to ad prompt with two-step UI

refactor: extract coach modal animation into reusable components

feat: Change coach ad prompt threshold from 15 to 6 messages and hide UI elements

feat: add home button in menu

refactor: Remove studentName/coachName from database, derive from templates

feat: Add comprehensive Basic Auth and enhance prompts with developmental theory

feat: Add delete conversation feature with confirmation dialog

feat: Improve sidebar UX by displaying action buttons inline

feat: Add auto-generated conversation titles with inline editing

feat: Add smooth modal animations to sidebar

feat: Auto-navigate to latest conversation and fix logout flow

feat: Add ChatGPT-style hamburger menu navigation with smooth animations

feat: Complete SEL Chat Coach - Traditional Chinese social-emotional learning platform