[Good First Issue] Add dark mode support to the web dashboard

[pixelstrade-dev]

Description

The CAIMS dashboard only supports light mode. Adding dark mode would improve UX during long evaluation sessions.

What to do

1. Add a theme toggle button in the navigation bar
2. 2. Create CSS variables for light/dark themes
3. 3. Implement theme persistence via localStorage
4. 4. Update all components to use theme variables
5. 5. Add system preference detection (prefers-color-scheme)

Skills needed

• React / Next.js
• • CSS / Tailwind CSS
• • • Basic TypeScript

References

• Main layout: apps/web/app/layout.tsx
• • Dashboard: apps/web/app/page.tsx