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/LoadingSpinner.razor

@@ -0,0 +1,17 @@
+@* Reusable loading spinner *@
+
+@if (IsLoading)
+{
+    <div class="d-flex align-items-center text-muted @CssClass">
+        <div class="spinner-border spinner-border-sm me-2" role="status">
+            <span class="visually-hidden">Loading...</span>
+        </div>
+        <span>@Message</span>
+    </div>
+}
+
+@code {
+    [Parameter] public bool IsLoading { get; set; }
+    [Parameter] public string Message { get; set; } = "Loading...";
+    [Parameter] public string CssClass { get; set; } = "";
+}