feat: add notification bell with activity feed and notification preferences

- New NotificationBell component in topbar showing recent activity
- Popover with last 10 activity items from /api/domains/[id]/activity
- Notification preferences section in Settings > Shortcuts tab
- Preferences persisted to localStorage (pe_notification_prefs)
- Toggle switches for task_created, task_completed, habit_logged, comment_added

apps/web/components/topbar.tsx

@@ -42,9 +42,7 @@ export function TopBar() {
             <Plus className="mr-1 h-4 w-4" aria-hidden="true" />
             {label}
           </Button>
-          <Button variant="ghost" size="icon" aria-label="Notifications">
-            <Bell className="h-5 w-5" aria-hidden="true" />
-          </Button>
+          <NotificationBell />
         </div>
       </header>
       <CommandPalette />