Phases 4-6: Complete Central UI — Admin, Design, Deployment, and Operations pages

Phase 4 — Operator/Admin UI:
- Sites, DataConnections, Areas (hierarchical), API Keys (auto-generated) CRUD
- Health Dashboard (live refresh, per-site metrics from CentralHealthAggregator)
- Instance list with filtering/staleness/lifecycle actions
- Deployment status tracking with auto-refresh

Phase 5 — Authoring UI:
- Template authoring with inheritance tree, tabs (attrs/alarms/scripts/compositions)
- Lock indicators, on-demand validation, collision detection
- Shared scripts with syntax check
- External systems, DB connections, notification lists, Inbound API methods

Phase 6 — Deployment Operations UI:
- Staleness indicators, validation gating
- Debug view (instance selection, attribute/alarm live tables)
- Site event log viewer (filters, keyword search, keyset pagination)
- Parked message management, Audit log viewer with JSON state

Shared components: DataTable, ConfirmDialog, ToastNotification, LoadingSpinner, TimestampDisplay
623 tests pass, zero warnings. All Bootstrap 5, clean corporate design.

src/ScadaLink.CentralUI/Components/Shared/TimestampDisplay.razor

@@ -0,0 +1,8 @@
+@* Displays a UTC DateTimeOffset formatted for display. Tooltip shows UTC value. *@
+
+<span title="@Value.UtcDateTime.ToString("yyyy-MM-dd HH:mm:ss") UTC">@Value.LocalDateTime.ToString(Format)</span>
+
+@code {
+    [Parameter, EditorRequired] public DateTimeOffset Value { get; set; }
+    [Parameter] public string Format { get; set; } = "yyyy-MM-dd HH:mm:ss";
+}