feat: add Notification Settings Modal and Stack Alert Sheet components

- Implemented NotificationSettingsModal for configuring notification agents (Discord, Slack, Webhook) and global settings.
- Added StackAlertSheet for managing stack-specific alert rules with metrics, thresholds, and cooldowns.
- Introduced reusable UI components: DropdownMenu, Popover, Select, Sheet, Slider, and Switch.
- Enhanced Tabs component by removing unnecessary client directive.

backend/package.json

@@ -15,6 +15,7 @@
   "type": "commonjs",
   "devDependencies": {
     "@types/bcrypt": "^6.0.0",
+    "@types/better-sqlite3": "^7.6.13",
     "@types/cookie-parser": "^1.4.10",
     "@types/jsonwebtoken": "^9.0.10",
     "@types/node": "^25.3.0",
@@ -29,6 +30,7 @@
     "@types/express": "^5.0.6",
     "@types/ws": "^8.18.1",
     "bcrypt": "^6.0.0",
+    "better-sqlite3": "^12.6.2",
     "composerize": "^1.7.5",
     "cookie-parser": "^1.4.7",
     "cors": "^2.8.6",