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/ConfirmDialog.razor

@@ -0,0 +1,55 @@
+@* Reusable confirmation dialog using Bootstrap modal *@
+
+@if (_visible)
+{
+    <div class="modal-backdrop fade show"></div>
+    <div class="modal fade show d-block" tabindex="-1" role="dialog">
+        <div class="modal-dialog modal-dialog-centered" role="document">
+            <div class="modal-content">
+                <div class="modal-header">
+                    <h5 class="modal-title">@Title</h5>
+                    <button type="button" class="btn-close" @onclick="Cancel"></button>
+                </div>
+                <div class="modal-body">
+                    <p>@Message</p>
+                </div>
+                <div class="modal-footer">
+                    <button type="button" class="btn btn-outline-secondary btn-sm" @onclick="Cancel">Cancel</button>
+                    <button type="button" class="btn @ConfirmButtonClass btn-sm" @onclick="Confirm">@ConfirmText</button>
+                </div>
+            </div>
+        </div>
+    </div>
+}
+
+@code {
+    private bool _visible;
+    private TaskCompletionSource<bool>? _tcs;
+
+    [Parameter] public string Title { get; set; } = "Confirm";
+    [Parameter] public string Message { get; set; } = "Are you sure?";
+    [Parameter] public string ConfirmText { get; set; } = "Confirm";
+    [Parameter] public string ConfirmButtonClass { get; set; } = "btn-danger";
+
+    public Task<bool> ShowAsync(string? message = null, string? title = null)
+    {
+        if (message != null) Message = message;
+        if (title != null) Title = title;
+        _visible = true;
+        _tcs = new TaskCompletionSource<bool>();
+        StateHasChanged();
+        return _tcs.Task;
+    }
+
+    private void Confirm()
+    {
+        _visible = false;
+        _tcs?.TrySetResult(true);
+    }
+
+    private void Cancel()
+    {
+        _visible = false;
+        _tcs?.TrySetResult(false);
+    }
+}