fix(health-monitoring): resolve HealthMonitoring-015 — nullable LastReportReceivedAt

A heartbeat-registered site that has never sent a full report now has
LastReportReceivedAt = null instead of the year-0001 sentinel. TimestampDisplay
accepts DateTimeOffset? and renders null as a placeholder ('awaiting first
report') rather than a ~2000-year-stale date. Cross-module: HealthMonitoring +
CentralUI.

tests/ScadaLink.CentralUI.Tests/Shared/TimestampDisplayTests.cs

@@ -0,0 +1,50 @@
+using Bunit;
+using ScadaLink.CentralUI.Components.Shared;
+
+namespace ScadaLink.CentralUI.Tests.Shared;
+
+/// <summary>
+/// Regression tests for HealthMonitoring-015. A heartbeat-only registered site has
+/// a <c>null</c> <c>LastReportReceivedAt</c> ("no full report yet"). The health
+/// dashboard passes that value straight into <see cref="TimestampDisplay"/>, so the
+/// component's <c>Value</c> must accept <c>DateTimeOffset?</c> and render a
+/// <c>null</c> as a human-readable placeholder ("never") instead of the
+/// <c>DateTimeOffset.MinValue</c> year-0001 sentinel. Non-null callers must keep
+/// rendering the formatted timestamp exactly as before.
+/// </summary>
+public class TimestampDisplayTests : BunitContext
+{
+    [Fact]
+    public void Render_NonNullValue_ShowsFormattedTimestamp()
+    {
+        var value = new DateTimeOffset(2026, 5, 17, 14, 30, 45, TimeSpan.Zero);
+
+        var cut = Render<TimestampDisplay>(parameters => parameters
+            .Add(p => p.Value, (DateTimeOffset?)value)
+            .Add(p => p.Format, "HH:mm:ss"));
+
+        var span = cut.Find("span");
+        Assert.Equal(value.LocalDateTime.ToString("HH:mm:ss"), span.TextContent.Trim());
+        Assert.Contains("2026-05-17 14:30:45 UTC", span.GetAttribute("title")!);
+    }
+
+    [Fact]
+    public void Render_NullValue_ShowsNeverPlaceholder()
+    {
+        var cut = Render<TimestampDisplay>(parameters => parameters
+            .Add(p => p.Value, (DateTimeOffset?)null)
+            .Add(p => p.Format, "HH:mm:ss"));
+
+        Assert.Contains("never", cut.Markup, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void Render_NullValue_DoesNotRenderYear0001Sentinel()
+    {
+        var cut = Render<TimestampDisplay>(parameters => parameters
+            .Add(p => p.Value, (DateTimeOffset?)null));
+
+        // The year-0001 DateTimeOffset.MinValue sentinel must never reach the UI.
+        Assert.DoesNotContain("0001", cut.Markup);
+    }
+}