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/api/client.ts

@@ -0,0 +1,66 @@
+import { de } from '../strings/de'
+
+/**
+ * Basis-URL der GerbilManagerWebAPI.
+ * Per VITE_API_BASE_URL konfigurierbar (z. B. in Docker / Aspire);
+ * Standard ist das lokale Dev-Profil der API (launchSettings.json, Profil "http").
+ */
+export const API_BASE_URL: string =
+  import.meta.env.VITE_API_BASE_URL ?? 'http://localhost:5179'
+
+export class ApiError extends Error {
+  readonly status: number | null
+
+  constructor(message: string, status: number | null) {
+    super(message)
+    this.name = 'ApiError'
+    this.status = status
+  }
+}
+
+function errorMessageFor(status: number): string {
+  if (status === 404) return de.api.errors.notFound
+  if (status >= 500) return de.api.errors.server
+  return de.api.errors.unknown
+}
+
+async function request<T>(path: string, init?: RequestInit): Promise<T> {
+  let response: Response
+  try {
+    response = await fetch(`${API_BASE_URL}${path}`, {
+      headers: { 'Content-Type': 'application/json', ...init?.headers },
+      ...init,
+    })
+  } catch {
+    throw new ApiError(de.api.errors.network, null)
+  }
+
+  if (!response.ok) {
+    throw new ApiError(errorMessageFor(response.status), response.status)
+  }
+
+  if (response.status === 204) {
+    return undefined as T
+  }
+  return (await response.json()) as T
+}
+
+export const api = {
+  get: <T>(path: string) => request<T>(path),
+  post: <T>(path: string, body: unknown) =>
+    request<T>(path, { method: 'POST', body: JSON.stringify(body) }),
+  put: <T>(path: string, body: unknown) =>
+    request<T>(path, { method: 'PUT', body: JSON.stringify(body) }),
+  delete: (path: string) => request<void>(path, { method: 'DELETE' }),
+}
+
+/**
+ * Ressourcen-Pfade der API (siehe GerbilManagerWebAPI/Controllers).
+ * Alle drei Ressourcen bieten volle CRUD-Endpunkte:
+ * GET /{resource}, GET /{resource}/{id}, POST, PUT /{id}, DELETE /{id}
+ */
+export const resources = {
+  gerbils: '/gerbils',
+  litters: '/litters',
+  breeders: '/breeders',
+} as const