fix(alerts): overhaul alerts & notifications system for local and remote nodes

- StackAlertSheet: fetch agent status on open and show contextual banner
  (amber warning when no channels configured, green when active, blue info
  callout on remote nodes explaining remote-instance evaluation semantics)
- StackAlertSheet: surface actual server error message on addAlert failure
  instead of generic string; add console logging for all failure paths
- SettingsModal: expose Notifications tab for remote nodes so agents can be
  configured directly on any Sencho instance; section header shows node name
  and Remote badge with tooltip when proxying to a remote node
- SettingsModal: re-fetch agents/settings when active node changes
- SettingsModal: add hidden DialogTitle/DialogDescription to satisfy Radix
  UI accessibility requirements (eliminates console errors)
- backend POST /api/alerts: add Zod validation schema; rejects unknown
  metric/operator values, negative thresholds, and missing fields with 400
- EditorLayout WS: upgrade reconnect from flat 5s retry to exponential
  backoff (1s→2s→4s→8s→16s→30s max); onerror now logs the event; cleanup
  only closes OPEN sockets — CONNECTING sockets are closed in onopen after
  isMounted check, eliminating "closed before established" StrictMode error

backend/src/index.ts

@@ -1392,12 +1392,26 @@ app.get('/api/alerts', async (req: Request, res: Response) => {
   }
 });
 
+const AlertCreateSchema = z.object({
+  stack_name: z.string().min(1).max(255),
+  metric: z.enum(['cpu_percent', 'memory_percent', 'memory_mb', 'net_rx', 'net_tx', 'restart_count']),
+  operator: z.enum(['>', '>=', '<', '<=', '==']),
+  threshold: z.number().min(0),
+  duration_mins: z.coerce.number().int().min(0).max(1440),
+  cooldown_mins: z.coerce.number().int().min(0).max(10080),
+});
+
 app.post('/api/alerts', async (req: Request, res: Response) => {
+  const parsed = AlertCreateSchema.safeParse(req.body);
+  if (!parsed.success) {
+    res.status(400).json({ error: 'Invalid alert data', details: parsed.error.flatten().fieldErrors });
+    return;
+  }
   try {
-    const alert = req.body;
-    DatabaseService.getInstance().addStackAlert(alert);
+    DatabaseService.getInstance().addStackAlert(parsed.data);
     res.json({ success: true });
   } catch (error) {
+    console.error('Failed to add alert:', error);
     res.status(500).json({ error: 'Failed to add alert' });
   }
 });