Phase 2: monitoring warnings, dashboard alerts & notification system

- Monitoring page: "Távoli monitoring" section showing healthcheck ping UUID
  configuration status (configured/not configured) for each of the 5 pings
- Alert manager: persistent dashboard banners on all pages generated from
  health check results, missing pings, and backup status
- Notification system: controller-side notifier sends events to hub relay,
  with cooldown tracking and event-type filtering
- Notification preferences UI: email, event checkboxes, cooldown settings
  on the settings page with test email functionality
- Settings refactored: shared settingsData() helper, NotificationPrefs
  struct with getter/setter and defaults

New files:
- controller/internal/web/alerts.go (AlertManager)
- controller/internal/notify/notifier.go (hub notification client)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/web/templates/layout.html

@@ -28,6 +28,17 @@
         </div>
     </nav>
     <main class="content">
+{{if .Alerts}}
+<div class="alerts-container">
+    {{range .Alerts}}
+    <div class="alert-banner alert-banner-{{.Level}}">
+        <span class="alert-icon">{{if eq .Level "error"}}🔴{{else if eq .Level "warning"}}🟡{{else}}ℹ️{{end}}</span>
+        <span class="alert-message">{{.Message}}</span>
+        {{if .Link}}<a href="{{.Link}}" class="alert-link">{{.LinkText}} →</a>{{end}}
+    </div>
+    {{end}}
+</div>
+{{end}}
 {{end}}
 
 {{define "layout_end"}}