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)

app/plugins/builtins/kommunikation/plugin.py

@@ -6,7 +6,7 @@ import logging
 from typing import Any
 
 from app.plugins.base import BasePlugin
-from app.plugins.manifest import PluginManifest, PluginRouteDef
+from app.plugins.manifest import PluginManifest, PluginRouteDef, FrontendMenuItem, FrontendPageRoute
 
 logger = logging.getLogger(__name__)
 
@@ -49,6 +49,12 @@ class KommunikationPlugin(BasePlugin):
             "comm:delete",
         ],
         is_core=True,
+        menu_items=[
+            FrontendMenuItem(label_key='nav.communication', label='Communication', path='/communication', icon='MessageSquare', order=80),
+        ],
+        page_routes=[
+            FrontendPageRoute(path='/communication', component='@/pages/Communication', protected=True),
+        ],
     )
 
     async def on_activate(self, db, service_container, event_bus) -> None: