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/components/layout/TopBar.tsx

@@ -9,6 +9,7 @@ import { Avatar } from '@/components/ui/Avatar';
 import { SearchDropdown } from '@/components/shared/SearchDropdown';
 import { SuggestionBadge } from '@/components/ai/SuggestionBadge';
 import { Building, ChevronDown, Menu, Zap, Bot, Layers } from 'lucide-react';
+import { NotificationBell } from '@/components/layout/NotificationBell';
 import { useWindowStore } from '@/store/windowStore';
 
 export function TopBar() {
@@ -80,6 +81,7 @@ export function TopBar() {
       </div>
 
       <div className="flex items-center gap-2">
+        <NotificationBell />
         {/* Minimized windows */}
         {minimizedWindows.length > 0 && (
           <div className="flex items-center gap-1.5">