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/components/plugins/PluginRegistry.tsx

@@ -0,0 +1,35 @@
+import { useEffect } from 'react';
+import { useActivePluginManifests } from '@/api/pluginManifests';
+import { usePluginStore } from '@/store/pluginStore';
+
+/**
+ * PluginRegistry — invisible provider component that fetches active plugin
+ * UI manifests on mount and populates the plugin store.
+ *
+ * Must be placed inside AppShell or at the root of the protected route tree.
+ */
+export function PluginRegistry() {
+  const { data, isLoading, error } = useActivePluginManifests();
+  const setManifests = usePluginStore((s) => s.setManifests);
+  const setLoading = usePluginStore((s) => s.setLoading);
+  const setError = usePluginStore((s) => s.setError);
+
+  useEffect(() => {
+    setLoading(isLoading);
+  }, [isLoading, setLoading]);
+
+  useEffect(() => {
+    if (data?.plugins) {
+      setManifests(data.plugins);
+    }
+  }, [data, setManifests]);
+
+  useEffect(() => {
+    if (error) {
+      setError(error instanceof Error ? error.message : 'Failed to load plugin manifests');
+    }
+  }, [error, setError]);
+
+  // This component renders nothing visible
+  return null;
+}