feat: implement ChatWindow component with document upload, voice transcription, and streaming responses

feat: implement premium dark mode theme and Sidebar component structure

feat: implement Midnight Amethyst theme, ChatWindow UI component, and backend entry point

clean: remove single-document delete option from library view

style: fix mobile header notch overlap and chat bubble citation overflow

style: fix library table actions column overflow on desktop

feat: implement full-stack authentication, chat interface, document management, and voice service integration

Initial clean commit