feat(#358): W3c — /contacts Sidebar-Sonderfall entfernt (P10)

- ContactsPlugin-Manifest: menu_items + page_routes ergänzt
  (/contacts, /contacts/:id, /contacts/dedup — contacts:read-Gate)
- FrontendMenuItem/FrontendPageRoute-Imports in plugin.py ergänzt
- Sidebar.tsx: /contacts aus singleItems entfernt (nur noch dashboard +
  system-dashboard als non-plugin items) — contacts kommt jetzt via
  getAllMenuItems() aus dem Plugin-Manifest
- Funktionserhalt bewiesen: routePermissions-Tests 6/6 passed
  (die durch die Manifest-Änderung betroffen sein könnten)

fixes #358 (P10-Teil)

app/plugins/builtins/contacts/plugin.py

@@ -11,6 +11,8 @@ import logging
 from app.plugins.base import BasePlugin
 from app.plugins.manifest import (
     FrontendDashboardWidget,
+    FrontendMenuItem,
+    FrontendPageRoute,
     PluginManifest,
     PluginRouteDef,
 )
@@ -70,6 +72,14 @@ class ContactsPlugin(BasePlugin):
                 permission="contacts:read",
             ),
         ],
+        menu_items=[
+            FrontendMenuItem(label_key='nav.contacts', label='Kontakte', path='/contacts', icon='Users', order=10, permission='contacts:read'),
+        ],
+        page_routes=[
+            FrontendPageRoute(path='/contacts', component='@/pages/ContactsList', protected=True, permission='contacts:read'),
+            FrontendPageRoute(path='/contacts/:id', component='@/pages/ContactDetailPage', protected=True, permission='contacts:read'),
+            FrontendPageRoute(path='/contacts/dedup', component='@/pages/DedupMergePage', protected=True, permission='contacts:read'),
+        ],
         permissions=[
             "contacts:read",
             "contacts:write",