feat(chatbot-ui): modernize OpenCRE chatbot experience (stacked on #795)

[PRAteek-singHWY]

Summary

This PR modernizes the OpenCRE chatbot UI/UX while keeping existing chatbot functionality and data flow intact.

Stacked PR Context

• This PR is stacked on: feat(chatbot): separate answer instructions from prompt matching (#376) #795
• Base branch for this PR: fix/376-separate-answer-language-input
• Please merge/review feat(chatbot): separate answer instructions from prompt matching (#376) #795 first, then this PR
• Fixes Have a separate UI element to enter language request, to prevent requests in the prompt to bother the matching #376

What Changed

• Redesigned chatbot layout with modern, production-style UI
• Improved chat message presentation and conversation flow
• Added instruction presets and improved composer ergonomics
• Improved mobile responsiveness and spacing behavior
• Added better scroll-to-latest interaction placement/styling
• Improved frontend error display for HTML 500 responses (clean message instead of raw HTML)

Scope Safety

• Only UI files are changed in this PR:
  • application/frontend/src/pages/chatbot/chatbot.tsx
  • application/frontend/src/pages/chatbot/chatbot.scss

Validation

• yarn build passes (existing webpack size warnings only)

Screenshots / Video

Before

After

Screen.Recording.2026-03-07.at.15.50.53.mov