feat: add Unsplash search hook and API proxy routes

- Implemented `useUnsplashSearch` hook for searching Unsplash photos with debounce.
- Created Unsplash API client functions for searching photos and triggering downloads.
- Added proxy routes for Unsplash API to handle search requests and background image uploads.
- Introduced accessibility utilities for WCAG compliance checks and motion preference management.
- Developed theme sharing module for encoding and decoding theme configurations as base64url strings.

ccw/frontend/src/components/codexlens/ModelsTab.tsx

@@ -47,7 +47,7 @@ function filterModels(models: CodexLensModel[], filter: FilterType, search: stri
     filtered = filtered.filter(m =>
       m.name.toLowerCase().includes(query) ||
       m.profile.toLowerCase().includes(query) ||
-      m.backend.toLowerCase().includes(query)
+      (m.description?.toLowerCase().includes(query) ?? false)
     );
   }