feat(ui): health dashboard shows metrics-stale badge and offline-since timestamp

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Monitoring/Health.razor

@@ -18,6 +18,7 @@
 @inject CommunicationService CommunicationService
 @inject IAuditLogQueryService AuditLogQueryService
 @inject IKpiHistoryQueryService KpiHistory
+@inject Microsoft.Extensions.Options.IOptions<HealthMonitoringOptions> HealthOptions
 
 <div class="container-fluid mt-3">
     <div class="d-flex justify-content-between align-items-center mb-3">
@@ -199,6 +200,17 @@
                             <span class="badge bg-danger me-2" aria-label="State: Offline">@OfflineGlyph Offline</span>
                         }
                         <strong class="fs-5">@siteName@(isCentral ? "" : $" ({siteId})")</strong>
+                        @if (state.IsOnline && state.IsMetricsStale)
+                        {
+                            <span class="badge bg-warning text-dark ms-2"
+                                  title="Site is heartbeating but has sent no health report for over @StaleTimeoutDisplay — the metrics pipeline may be dead.">
+                                Metrics stale
+                            </span>
+                        }
+                        @if (!state.IsOnline && state.LastStatusChangeAt is { } changedAt)
+                        {
+                            <small class="text-muted ms-2">offline since @changedAt.ToString("u")</small>
+                        }
                     </div>
                     <small class="text-muted">
                         Last report: <TimestampDisplay Value="@state.LastReportReceivedAt" Format="HH:mm:ss" NullText="awaiting first report" />
@@ -419,6 +431,16 @@
     private const string PrimaryGlyph = "▲";  // ▲
     private const string StandbyGlyph = "△";  // △
 
+    // Human-friendly rendering of the configured metrics-stale window for the
+    // "Metrics stale" badge tooltip (e.g. "2 minutes").
+    private string StaleTimeoutDisplay =>
+        FormatDuration(HealthOptions.Value.MetricsStaleTimeout);
+
+    private static string FormatDuration(TimeSpan span) =>
+        span.TotalMinutes >= 1 && span == TimeSpan.FromMinutes(Math.Round(span.TotalMinutes))
+            ? $"{span.TotalMinutes:0} minute{(span.TotalMinutes == 1 ? "" : "s")}"
+            : $"{span.TotalSeconds:0} seconds";
+
     private IReadOnlyDictionary<string, SiteHealthState> _siteStates = new Dictionary<string, SiteHealthState>();
     private Dictionary<string, string> _siteNames = new();
     private Timer? _refreshTimer;