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/src/App.tsx

@@ -0,0 +1,23 @@
+import { BrowserRouter, Route, Routes } from 'react-router-dom'
+import AppShell from './components/AppShell'
+import BreedersPage from './pages/BreedersPage'
+import GerbilsPage from './pages/GerbilsPage'
+import HomePage from './pages/HomePage'
+import LittersPage from './pages/LittersPage'
+import NotFoundPage from './pages/NotFoundPage'
+
+export default function App() {
+  return (
+    <BrowserRouter>
+      <Routes>
+        <Route element={<AppShell />}>
+          <Route index element={<HomePage />} />
+          <Route path="rennmaeuse" element={<GerbilsPage />} />
+          <Route path="wuerfe" element={<LittersPage />} />
+          <Route path="zuechter" element={<BreedersPage />} />
+          <Route path="*" element={<NotFoundPage />} />
+        </Route>
+      </Routes>
+    </BrowserRouter>
+  )
+}