feat(web): add light mode theme support

[zhangweijian97]

Summary

Add light mode (day theme) support to the Web UI, with design reference from Linear's Light Mode.

Current State

The Web UI currently uses a hardcoded dark theme. The CSS variable system is already in place (HSL format in globals.css), but there is no theme switching mechanism (no next-themes or similar).

Requirements

1. Light mode color palette inspired by Linear's Light Mode:

   • Warm gray tones, avoiding pure white + pure black contrast
   • Clean, professional look
   • Default to light mode (user's preference due to astigmatism)

2. Theme switching mechanism:

   • Add a theme toggle in the UI
   • Persist user preference (localStorage)
   • Consider using next-themes for Next.js integration

3. Areas to update:

   • CSS variables in globals.css (already HSL, good foundation)
   • Replace hardcoded text-zinc-*, bg-zinc-* classes (~10-20 occurrences)
   • Ensure all components respect the theme

Design Reference

• Linear Light Mode: warm gray tones, subtle borders, clean typography
• Avoid harsh pure white backgrounds — use warm off-white tones
• Ensure sufficient contrast for readability

Technical Notes

• The CSS variable system is well-structured with HSL values — this is a good foundation for dual-theme support
• Estimated effort: 2-3 hours
• No backend changes needed