docs: add development setup guides and improve UI components

[flanzipit]

Summary

This PR enhances the Chainlit development experience and improves several UI components. The changes include comprehensive developer onboarding documentation, UI/UX improvements to sharing functionality, better thread management, and enhanced configuration handling.

Technical Implementation Details

Documentation Enhancements

• WARP.md: Added comprehensive setup guide for WARP.dev integration with step-by-step instructions
• copilot-instructions.md: Created detailed developer onboarding documentation with prerequisites and essential commands

Frontend Improvements

• ShareDialog: Enhanced UI layout with improved modal functionality and better user experience
• ThreadList: Improved thread management with better state handling and user interaction
• API Client: Refactored methods for improved error handling and more robust HTTP requests
• Configuration: Enhanced hooks (useChatSession.ts, useConfig.ts) for better state management
• Styling: Updated CSS for improved responsive design and visual consistency

Testing Infrastructure

• Cypress: Improved E2E test stability with better path resolution and port propagation
• Build Process: Enhanced build validation with proper entrypoint checking

Testing Steps

1. Development Setup:

   # Test new setup instructions
   cd backend && uv sync --extra tests --extra mypy --extra dev
   cd ../frontend && pnpm install && pnpm dev

2. UI Functionality:

   • Test ShareDialog modal functionality and responsiveness
   • Verify ThreadList operations (create, edit, delete threads)
   • Check thread sharing workflow end-to-end

3. E2E Testing:

   pnpm test  # Run Cypress tests with improved stability

Files Changed

• WARP.md, copilot-instructions.md (new documentation)
• frontend/src/components/share/ShareDialog.tsx (UI improvements)
• frontend/src/components/LeftSidebar/ThreadList.tsx (thread management)
• libs/react-client/src/api/index.tsx (API client refactoring)
• Various configuration and styling updates

[asvishnyakov]

@flanzipit

1. We already have Copilot instructions
2. I don't see any difference in changed frontend files, except linting/formatting. I see some changes only in Cypress files.

Is this PR AI generated? If so, please carefully review it yourself, remove all unnecessary changes from both code and PR description and split it into a few separate (docs, frontend refactoring)