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>

libs/ui/vite.config.ts

@@ -40,7 +40,6 @@ export default defineConfig(() => ({
             'react',
             'react-dom',
             'react/jsx-runtime',
-            'react-router-dom',
             '@ant-design/icons',
             '@imphnen-frontend-service/service',
             '@imphnen-frontend-service/utils',