feat(dashboard): alerts page + toast notifications + polish

- Full Alerts page with active alerts, history table, subscriptions
- Active alert cards with severity styling and acknowledge button
- Alert history table with type/severity filtering and pagination
- Subscription viewer showing mesh subscriptions
- ToastProvider for app-wide toast notifications
- Toast notifications triggered on WebSocket alert_fired messages
- Auto-dismiss toasts after 8 seconds, click to navigate
- Page titles on all pages (Dashboard/Mesh/Environment/Config/Alerts)
- Improved alert_routes.py with proper pending alert handling
- Added AlertHistoryItem, Subscription types to api.ts
- Added fetchAlertHistory, fetchSubscriptions functions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

dashboard-frontend/src/pages/Environment.tsx

@@ -369,6 +369,7 @@ export default function Environment() {
   const [error, setError] = useState<string | null>(null)
 
   useEffect(() => {
+    document.title = 'Environment — MeshAI'
     Promise.all([
       fetchEnvStatus().catch(() => null),
       fetchEnvActive().catch(() => []),