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/index.css

@@ -47,3 +47,28 @@ body {
 .animate-pulse-slow {
   animation: pulse 2s cubic-bezier(0.4, 0, 0.6, 1) infinite;
 }
+
+
+/* Toast slide-in animation */
+@keyframes slide-in {
+  from {
+    transform: translateX(100%);
+    opacity: 0;
+  }
+  to {
+    transform: translateX(0);
+    opacity: 1;
+  }
+}
+
+.animate-slide-in {
+  animation: slide-in 0.3s ease-out;
+}
+
+/* Line clamp utility */
+.line-clamp-2 {
+  display: -webkit-box;
+  -webkit-line-clamp: 2;
+  -webkit-box-orient: vertical;
+  overflow: hidden;
+}