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>

gerbil-manager-web/README.md

@@ -0,0 +1,33 @@
+# gerbil-manager-web
+
+Frontend des Rennmaus-Managers — React + TypeScript + Vite, mobile-first (Smartphone und Laptop).
+
+## Entwicklung
+
+```bash
+npm install
+npm run dev
+```
+
+Erwartet die laufende GerbilManagerWebAPI unter `http://localhost:5179`
+(Profil `http` in `GerbilManagerWebAPI/Properties/launchSettings.json`).
+
+## Konfiguration
+
+| Variable | Standard | Beschreibung |
+|---|---|---|
+| `VITE_API_BASE_URL` | `http://localhost:5179` | Basis-URL der GerbilManagerWebAPI |
+
+## Konventionen
+
+- **Alle für Benutzer sichtbaren Texte sind Deutsch** und liegen zentral in
+  `src/strings/de.ts` — keine Texte direkt in Komponenten hartkodieren.
+- API-Zugriffe laufen über `src/api/client.ts`.
+- Routen werden in `src/App.tsx` registriert, Seiten liegen in `src/pages/`.
+
+## Build
+
+```bash
+npm run build
+npm run preview
+```