feat: add REST/WebSocket API layer, Tailwind UI and API tests

[Copilot]

This pull request introduces a significant backend and frontend update to support document upload and management, as well as a streaming chat API and a new, modernised React UI.
The backend adds REST endpoints for document management, a WebSocket endpoint for streaming chat, and centralises dependency injection.
The frontend is refactored to use custom hooks and Tailwind CSS for a more interactive and visually modern experience, including a sidebar for document uploads and a streaming chat interface.

Backend Improvements

• Document Management API
  • Added new REST endpoints in documents.py to upload, list, and delete documents, with in-memory metadata storage and file validation using allowed extensions from settings.
• Streaming Chat API
  • Introduced a WebSocket endpoint in chat_stream.py for token-by-token streaming chat responses, utilising dependency injection for the LLM client.
• Dependency Injection Refactoring
  • Centralised LLM client dependency with a new dependency module and updated chat endpoints to use it.
• Configuration Updates
  • Added settings for allowed upload extensions and WebSocket message size.
• API Routing and Prefixes
  • Updated API router to include new endpoints and removed the version prefix for consistency.

Frontend Improvements

• UI Redesign and Features
  • Refactored App.tsx to use Tailwind CSS, introduced a sidebar for document management, and modernised the chat interface.
  • Added a new DocumentUpload component for drag-and-drop uploads, progress, and document listing.
  • Updated ChatWindow to match the new design, support streaming, and improve message rendering.
• Codebase Cleanup
  • Removed unused Material UI imports and components, and switched to a custom, lighter UI approach.
• Miscellaneous
  • Updated .gitignore to exclude package-lock.json.