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/mail/plugin.py

@@ -7,7 +7,7 @@ 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, FrontendDetailTab, FrontendSettingsPage
 
 logger = logging.getLogger(__name__)
 
@@ -56,6 +56,20 @@ class MailPlugin(BasePlugin):
         events=[],
         migrations=["0001_initial.sql", "0006_flag_type.sql", "0007_sync_queue.sql", "0008_sync_queue_deleted_at.sql", "0009_remove_mail_soft_delete.sql"],
         permissions=["mail:read", "mail:send", "mail:config", "mail:share", "mail:write", "mail:delete"],
+        menu_items=[
+            FrontendMenuItem(label_key='nav.email', label='Mail', path='/mail', icon='Mail', order=30),
+            FrontendMenuItem(label_key='nav.emailSettings', label='Mail Settings', path='/mail/settings', icon='Settings', group='nav.email', order=31),
+        ],
+        page_routes=[
+            FrontendPageRoute(path='/mail', component='@/pages/Mail', protected=True),
+            FrontendPageRoute(path='/mail/settings', component='@/pages/MailSettings', protected=True),
+        ],
+        settings_pages=[
+            FrontendSettingsPage(path='mail', label_key='settings.mail', label='Mail', component='@/pages/MailSettings', icon='Mail', order=50),
+        ],
+        detail_tabs=[
+            FrontendDetailTab(entity_type='contact', label_key='tabs.email', label='E-Mails', component='@/components/contact/ContactMailTab', icon='Mail', order=20, permission='mail:read'),
+        ],
     )
 
     async def on_activate(