feat: add keyboard shortcut support for chat actions

[kareena0229]

Summary

Implemented keyboard shortcut support for common chat actions to improve accessibility and productivity.

Added Shortcuts

• Ctrl + Enter → Send message
• Esc → Clear chat input
• Ctrl + K → Focus chat input
• Ctrl + Shift + C → Copy latest assistant response

Changes Made

• Added global keyboard listener using useEffect
• Added input focus support using inputRef
• Implemented latest assistant message copy functionality
• Prevented shortcut execution while typing in unrelated fields

Testing

• Verified shortcuts work on Dashboard chat panel
• Verified existing chat functionality remains unchanged
• npm run build passes successfully
• npm run lint passes with no new warnings introduced

• Frontend builds successfully
• Backend starts successfully
• Authentication flow tested successfully
• Document upload verified

Note: AI response generation could not be fully validated due to backend/RAG configuration issues unrelated to this PR.
Fixes #631

Screenshots

npm run build test

npm run lint tested

[kareena0229]

@param20h do review this PR

Testing Completed

• Successfully built the frontend using npm run build with no errors.
• Successfully started the backend using python -m uvicorn app.main:app --reload.
• Verified user authentication and login flow.
• Verified document upload functionality.
• Confirmed UI changes are working as expected and do not affect existing functionality.

Note: AI response generation could not be fully validated in the local environment due to backend/RAG configuration requirements (document/session authentication and model setup). This is unrelated to the changes introduced in this PR.

Screenshots and build verification have been attached for reference.