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/components/AppShell.tsx

@@ -0,0 +1,47 @@
+import { NavLink, Outlet } from 'react-router-dom'
+import { de } from '../strings/de'
+
+const navItems = [
+  { to: '/', label: de.nav.home, icon: '🏠', end: true },
+  { to: '/rennmaeuse', label: de.nav.gerbils, icon: '🐭', end: false },
+  { to: '/wuerfe', label: de.nav.litters, icon: '🍼', end: false },
+  { to: '/zuechter', label: de.nav.breeders, icon: '🧑‍🌾', end: false },
+]
+
+/**
+ * App-Grundgerüst, mobile-first:
+ * - Smartphone: Kopfzeile oben, Tab-Leiste unten
+ * - Laptop (ab 768px): Seitenleiste links, Inhalt rechts
+ */
+export default function AppShell() {
+  return (
+    <div className="app-shell">
+      <header className="app-header">
+        <span className="app-logo" aria-hidden="true">🐭</span>
+        <h1 className="app-title">{de.app.title}</h1>
+      </header>
+
+      <nav className="app-nav" aria-label={de.nav.mainNavigation}>
+        {navItems.map((item) => (
+          <NavLink
+            key={item.to}
+            to={item.to}
+            end={item.end}
+            className={({ isActive }) =>
+              isActive ? 'nav-link nav-link--active' : 'nav-link'
+            }
+          >
+            <span className="nav-icon" aria-hidden="true">
+              {item.icon}
+            </span>
+            <span className="nav-label">{item.label}</span>
+          </NavLink>
+        ))}
+      </nav>
+
+      <main className="app-main">
+        <Outlet />
+      </main>
+    </div>
+  )
+}