feat: add notification suppression rules (#1525)

* feat: add notification suppression rules

* fix: restore label routing and routing test mocks for suppression

* fix: allow bell mute shortcuts for history-only notification categories

Suppression rule validation used the routable category whitelist, which rejected history-only categories such as update_started that appear in the bell during stack updates.

* feat: expand Mute Rules UX with compose-first entry points and activity badges

* fix: add missing NodeContext mocks for notification suppression tests

backend/src/__tests__/notification-routing.test.ts

@@ -8,12 +8,14 @@ import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 
 const {
   mockGetEnabledNotificationRoutes,
+  mockGetEnabledNotificationSuppressionRules,
   mockGetEnabledAgents,
   mockGetStackLabelIds,
   mockAddNotificationHistory,
   mockUpdateNotificationDispatchError,
 } = vi.hoisted(() => ({
   mockGetEnabledNotificationRoutes: vi.fn().mockReturnValue([]),
+  mockGetEnabledNotificationSuppressionRules: vi.fn().mockReturnValue([]),
   mockGetEnabledAgents: vi.fn().mockReturnValue([]),
   mockGetStackLabelIds: vi.fn().mockReturnValue([]),
   mockAddNotificationHistory: vi.fn().mockReturnValue({
@@ -30,6 +32,7 @@ vi.mock('../services/DatabaseService', () => ({
   DatabaseService: {
     getInstance: () => ({
       getEnabledNotificationRoutes: mockGetEnabledNotificationRoutes,
+      getEnabledNotificationSuppressionRules: mockGetEnabledNotificationSuppressionRules,
       getEnabledAgents: mockGetEnabledAgents,
       getStackLabelIds: mockGetStackLabelIds,
       addNotificationHistory: mockAddNotificationHistory,