Phase 3: Plugin-UI-System (WordPress-Style)

Backend:
- PluginManifest um 5 neue UI-Felder erweitert: menu_items, page_routes,
  detail_tabs, settings_pages, dashboard_widgets (FrontendMenuItem,
  FrontendPageRoute, FrontendDetailTab, FrontendSettingsPage,
  FrontendDashboardWidget)
- GET /api/v1/plugins/active-manifests Endpoint liefert UI-Manifeste
  aller aktiven Plugins
- Registry.get_active_manifests() + PluginService.get_active_manifests()
- 12 Built-in Plugins mit UI-Manifest-Daten gefuellt (menu_items,
  page_routes, detail_tabs, settings_pages)
- Plugin-Install-System: POST /upload (ZIP), POST /install-url (URL)
  mit Validierung (Manifest, dangerous imports, SQL migrations)

Frontend:
- pluginStore.ts (Zustand) mit PluginUiManifest Typen + Selektoren
- useActivePluginManifests() React Query Hook
- PluginRegistry.tsx — fetcht Manifeste beim App-Start
- PluginLoader.tsx — dynamisches React.lazy() mit ErrorBoundary
- PluginRouteRenderer.tsx — Catch-all fuer Plugin-Routes
- routes/index.tsx — Catch-all Routes fuer Plugin-Pages + Settings
- Sidebar.tsx — dynamische Plugin Menu-Items mit Grouping + Icons
- Settings.tsx — dynamische Plugin Settings-Pages
- ContactDetail.tsx — dynamische Plugin Detail-Tabs mit Permissions
- AppShell.tsx — PluginRegistry Provider eingebunden
- SettingsPlugins.tsx — Install-UI (ZIP Upload + URL Install)
- plugins.ts — useUploadPlugin() + useInstallPluginFromUrl() Hooks

Docs & Templates:
- docs/plugin-development-guide.md — komplette Entwickler-Doku
- templates/plugin-template/ — Boilerplate mit allen Manifest-Feldern

Tests:
- 34 Vitest-Tests (PluginRegistry, PluginLoader, PluginRouteRenderer,
  pluginStore) — alle bestanden
- TSC: keine neuen Errors (nur pre-existing Dms.tsx)

frontend/src/routes/index.tsx

@@ -6,6 +6,7 @@ import { LoginPage } from '@/pages/Login';
 import { PasswordResetRequestPage } from '@/pages/PasswordResetRequest';
 import { PasswordResetConfirmPage } from '@/pages/PasswordResetConfirm';
 import { Loader2 } from 'lucide-react';
+import { PluginRouteRenderer } from '@/components/plugins/PluginRouteRenderer';
 
 // Lazy-loaded pages (code-splitting)
 const DashboardPage = React.lazy(() => import('@/pages/Dashboard').then(m => ({ default: m.DashboardPage })));
@@ -100,8 +101,10 @@ const router = createBrowserRouter([
           { path: 'ai', element: withSuspense(<AISettingsPage />) },
           { path: 'ai-proactive', element: withSuspense(<ProactiveAISettings />) },
           { path: 'theme', element: withSuspense(<SettingsThemePage />) },
+          { path: '*', element: <PluginRouteRenderer /> },
         ],
       },
+      { path: '*', element: <PluginRouteRenderer /> },
     ],
   },
 ]);