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/api/pluginManifests.ts

@@ -0,0 +1,15 @@
+import { apiGet } from './client';
+import { useQuery } from '@tanstack/react-query';
+import type { PluginUiManifest } from '../store/pluginStore';
+
+export function useActivePluginManifests() {
+  return useQuery({
+    queryKey: ['plugin-active-manifests'],
+    queryFn: async () => {
+      const res = await apiGet<{ plugins: PluginUiManifest[]; total: number }>('/plugins/active-manifests');
+      return res;
+    },
+    staleTime: 5 * 60 * 1000, // 5 min
+    refetchOnMount: true,
+  });
+}

frontend/src/api/plugins.ts

@@ -1,5 +1,5 @@
 /**
- * Plugin management hooks: list, install, activate, deactivate, uninstall.
+ * Plugin management hooks: list, install, activate, deactivate, uninstall, upload, install-url.
  */
 
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
@@ -65,3 +65,35 @@ export function useUninstallPlugin() {
     },
   });
 }
+
+export function useUploadPlugin() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: async (file: File) => {
+      const formData = new FormData();
+      formData.append('file', file);
+      const { default: apiClient } = await import('./client');
+      const res = await apiClient.post('/api/v1/plugins/upload', formData, {
+        headers: { 'Content-Type': 'multipart/form-data' },
+      });
+      return res.data;
+    },
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['plugins'] });
+    },
+  });
+}
+
+export function useInstallPluginFromUrl() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: async (url: string) => {
+      const { default: apiClient } = await import('./client');
+      const res = await apiClient.post('/api/v1/plugins/install-url', { url });
+      return res.data;
+    },
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['plugins'] });
+    },
+  });
+}