Replace Next.js app with Vite React app (gerbil-manager-web)

- Remove gerbil-manager-nextjs (unused scaffold; recoverable from history)
- Scaffold gerbil-manager-web: Vite + React 19 + TypeScript
- German-only UI (lang=de, central strings in src/strings/de.ts)
- Mobile-first shell: bottom tab bar on phones, sidebar on >=768px
- react-router with skeleton routes (Start, Rennmaeuse, Wuerfe, Zuechter)
- API client (src/api/client.ts) pointing at GerbilManagerWebAPI,
  configurable via VITE_API_BASE_URL (default http://localhost:5179)
- nginx-based Dockerfile with SPA fallback; docker-compose frontend
  service now builds gerbil-manager-web

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -5,10 +5,13 @@ version: '3.4'
 services:
 
   frontend:
-    image: gerbilmanagernextjs
+    image: gerbilmanagerweb
     build:
       context: .
-      dockerfile: gerbil-manager-nextjs/Dockerfile
+      dockerfile: gerbil-manager-web/Dockerfile
+      args:
+        # Der Browser erreicht die API über den am Host veröffentlichten Port
+        - VITE_API_BASE_URL=http://localhost:80
     ports:
      - 3000:3000
     networks: