feat(ui): (a) Backends + Routing-Rules + Settings pages + Sidebar

CRUD-Pages analog Domains:
* Backends: AntD Table + Modal-Form mit name, scheme, address, port,
  health_check_path, active. TanStack-Query gegen /api/v1/backends.
* RoutingRules: Table mit Domain-Name- und Backend-Label-Resolution,
  Modal mit Select-Pickern für Domain + Backend, Path-Prefix,
  Priority, Active. Drei parallele Queries (rules, domains, backends)
  liefern die Listen.
* Settings: read-only Descriptions-Cards mit /system/health und
  /setup/status. Editable Werte folgen später.

Sidebar erweitert um Backends, Routing-Rules, Settings (mit AntD-
Icons). i18n de/en für alle drei neuen Seiten.

bun run build + npx tsc -b strict (0 errors). Live-Smoke gegen API:
SPA-Routes /backends, /routing-rules, /settings antworten 200 mit
index.html (NoRoute fallback wirkt).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

management-ui/src/App.tsx

@@ -14,6 +14,9 @@ const LoginPage = lazy(() => import('./pages/Login'))
 const SetupPage = lazy(() => import('./pages/Setup'))
 const DashboardPage = lazy(() => import('./pages/Dashboard'))
 const DomainsPage = lazy(() => import('./pages/Domains'))
+const BackendsPage = lazy(() => import('./pages/Backends'))
+const RoutingRulesPage = lazy(() => import('./pages/RoutingRules'))
+const SettingsPage = lazy(() => import('./pages/Settings'))
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -74,6 +77,9 @@ export default function App() {
                   <Route path="/" element={<Navigate to="/dashboard" replace />} />
                   <Route path="/dashboard" element={<DashboardPage />} />
                   <Route path="/domains" element={<DomainsPage />} />
+                  <Route path="/backends" element={<BackendsPage />} />
+                  <Route path="/routing-rules" element={<RoutingRulesPage />} />
+                  <Route path="/settings" element={<SettingsPage />} />
                 </Route>
 
                 <Route path="*" element={<Navigate to="/dashboard" replace />} />