chore: remove dead code and unused dependencies

Deleted:
- libs/utils/src/react-router/ — old file-based routing system (replaced by TanStack Router)
- next-themes dependency (unused since Astro migration)
- react-router-dom from vite external config

Cleaned:
- Removed react-router barrel export from utils/index.ts
- Kept landing data files (still used by Astro components)
- Kept QueryProvider and ModalLoginProvider (still used by apps)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -59,7 +59,6 @@
     "graphql": "^16.13.2",
     "html2canvas": "^1.4.1",
     "js-cookie": "^3.0.5",
-    "next-themes": "^0.4.6",
     "openapi-fetch": "^0.17.0",
     "openapi-react-query": "^0.5.4",
     "picomatch": "^4.0.4",