Phase 3: Saved Filters UI, Entity History UI, Activity Timeline, API Docs Link

- Saved Filters: SavedFilterBar (dropdown, apply, delete), SaveFilterDialog (name + save)
- Entity History: EntityHistoryPanel (timeline, restore, undo), HistoryDiff (field changes visual)
- Activity Timeline: ActivityTimelinePage (grouped by day, pagination), ActivityFilter (user/entity/action/date)
- API Docs Link: TopBar user menu entry, SettingsSystem Entwickler section (Swagger, ReDoc, OpenAPI)
- Routes: /activity registered
- Menu items: Aktivitäten added to automation plugin manifest

frontend/src/routes/index.tsx

@@ -57,6 +57,7 @@ const DedupMergePage = React.lazy(() => import('@/pages/DedupMerge').then(m => (
 const ImportExportPage = React.lazy(() => import('@/pages/ImportExport').then(m => ({ default: m.ImportExportPage })));
 const TagsPage = React.lazy(() => import('@/pages/Tags').then(m => ({ default: m.TagsPage })));
 const CustomFieldsPage = React.lazy(() => import('@/pages/CustomFields').then(m => ({ default: m.CustomFieldsPage })));
+const ActivityTimelinePage = React.lazy(() => import('@/pages/ActivityTimeline').then(m => ({ default: m.ActivityTimelinePage })));
 
 /** Centered spinner fallback for lazy-loaded routes */
 function PageLoader() {
@@ -134,6 +135,7 @@ const router = createBrowserRouter([
       { path: '/contacts/dedup', element: withSuspense(<DedupMergePage />) },
       { path: '/import-export', element: withSuspense(<ImportExportPage />) },
       { path: '/tags', element: withSuspense(<TagsPage />) },
+      { path: '/activity', element: withSuspense(<ActivityTimelinePage />) },
       { path: '/profile', element: withSuspense(<SettingsProfilePage />) },
       {
         path: '/settings',