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>

README.md

@@ -1,3 +1,8 @@
+# GerbilManager
+
+- `GerbilManagerWebAPI/` — ASP.NET Core backend (Swagger unter `/swagger`)
+- `gerbil-manager-web/` — React-Frontend (Vite + TypeScript), siehe `gerbil-manager-web/README.md`
+
 # Requirements
 Install ef:
 
@@ -15,6 +20,11 @@ dotnet ef migrations add <name>
 ### force compose rebuild
 docker-compose build --no-cache
 
+## Frontend (gerbil-manager-web)
+cd gerbil-manager-web
+npm install
+npm run dev
+
 
 TODO:
 - Backup docker volume database