Phase 2: Tags UI, Custom Fields UI, Notifications Bell

- Tags UI: TagsPage (CRUD, color picker), TagBadge, TagSelector (multi-select, inline creation)
- Custom Fields Backend: model, schema, service, routes, migration 0041
- Custom Fields Frontend: CustomFieldsPage (definitions CRUD), CustomFieldRenderer (dynamic field rendering)
- Custom Fields: _collect_custom_field_definitions() extended to merge DB definitions with plugin definitions
- Notifications Bell: NotificationBell (30s polling, unread badge), NotificationDropdown, NotificationItem
- NotificationBell integrated into TopBar
- Routes: /tags, /settings/custom-fields registered
- Settings nav: Custom Fields entry added
- Menu items: Tags added to automation plugin manifest

frontend/src/pages/Settings.tsx

@@ -21,6 +21,7 @@ export function SettingsPage() {
     { to: '/settings/mail', label: t('mail.settings'), icon: '\ud83d\udce7' },
     { to: '/settings/ai-settings', label: 'KI Einstellungen', icon: '\ud83e\udde0' },
     { to: '/settings/notifications', label: t('settings.notifications'), icon: '\ud83d\udd14' },
+    { to: '/settings/custom-fields', label: 'Custom Fields', icon: '\ud83d\udccb' },
   ];
 
   const existingPaths = new Set(hardcodedNavItems.map(item => item.to));