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/Pages/Monitoring/ParkedMessages.razor

@@ -0,0 +1,153 @@
+@page "/monitoring/parked-messages"
+@attribute [Authorize]
+@using ScadaLink.Commons.Entities.Sites
+@using ScadaLink.Commons.Interfaces.Repositories
+@using ScadaLink.Commons.Messages.RemoteQuery
+@using ScadaLink.Communication
+@inject ISiteRepository SiteRepository
+@inject CommunicationService CommunicationService
+
+<div class="container-fluid mt-3">
+    <h4 class="mb-3">Parked Messages</h4>
+
+    <ToastNotification @ref="_toast" />
+    <ConfirmDialog @ref="_confirmDialog" />
+
+    <div class="row mb-3 g-2">
+        <div class="col-md-3">
+            <label class="form-label small">Site</label>
+            <select class="form-select form-select-sm" @bind="_selectedSiteId">
+                <option value="">Select site...</option>
+                @foreach (var site in _sites)
+                {
+                    <option value="@site.SiteIdentifier">@site.Name</option>
+                }
+            </select>
+        </div>
+        <div class="col-md-2 d-flex align-items-end">
+            <button class="btn btn-primary btn-sm" @onclick="Search"
+                    disabled="@(string.IsNullOrEmpty(_selectedSiteId) || _searching)">
+                @if (_searching) { <span class="spinner-border spinner-border-sm"></span> }
+                Query
+            </button>
+        </div>
+    </div>
+
+    @if (_errorMessage != null)
+    {
+        <div class="alert alert-danger">@_errorMessage</div>
+    }
+
+    @if (_messages != null)
+    {
+        <table class="table table-sm table-striped table-hover">
+            <thead class="table-dark">
+                <tr>
+                    <th>Message ID</th>
+                    <th>Target System</th>
+                    <th>Method</th>
+                    <th>Error</th>
+                    <th>Attempts</th>
+                    <th>Original</th>
+                    <th>Last Attempt</th>
+                    <th style="width: 120px;">Actions</th>
+                </tr>
+            </thead>
+            <tbody>
+                @if (_messages.Count == 0)
+                {
+                    <tr><td colspan="8" class="text-muted text-center">No parked messages.</td></tr>
+                }
+                @foreach (var msg in _messages)
+                {
+                    <tr>
+                        <td class="small"><code>@msg.MessageId[..Math.Min(12, msg.MessageId.Length)]</code></td>
+                        <td class="small">@msg.TargetSystem</td>
+                        <td class="small">@msg.MethodName</td>
+                        <td class="small text-danger">@msg.ErrorMessage</td>
+                        <td class="small text-center">@msg.AttemptCount</td>
+                        <td class="small"><TimestampDisplay Value="@msg.OriginalTimestamp" /></td>
+                        <td class="small"><TimestampDisplay Value="@msg.LastAttemptTimestamp" /></td>
+                        <td>
+                            <button class="btn btn-outline-success btn-sm py-0 px-1 me-1"
+                                    title="Retry message (not yet implemented)">Retry</button>
+                            <button class="btn btn-outline-danger btn-sm py-0 px-1"
+                                    title="Discard message (not yet implemented)">Discard</button>
+                        </td>
+                    </tr>
+                }
+            </tbody>
+        </table>
+
+        @if (_totalCount > 0)
+        {
+            <div class="d-flex justify-content-between align-items-center">
+                <span class="text-muted small">Page @_pageNumber of @((_totalCount + _pageSize - 1) / _pageSize) (@_totalCount total)</span>
+                <div>
+                    <button class="btn btn-outline-secondary btn-sm me-1" @onclick="PrevPage" disabled="@(_pageNumber <= 1)">Previous</button>
+                    <button class="btn btn-outline-secondary btn-sm" @onclick="NextPage" disabled="@(_messages.Count < _pageSize)">Next</button>
+                </div>
+            </div>
+        }
+    }
+</div>
+
+@code {
+    private List<Site> _sites = new();
+    private string _selectedSiteId = string.Empty;
+    private List<ParkedMessageEntry>? _messages;
+    private int _totalCount;
+    private int _pageNumber = 1;
+    private int _pageSize = 25;
+    private bool _searching;
+    private string? _errorMessage;
+
+    private ToastNotification _toast = default!;
+    private ConfirmDialog _confirmDialog = default!;
+
+    protected override async Task OnInitializedAsync()
+    {
+        _sites = (await SiteRepository.GetAllSitesAsync()).ToList();
+    }
+
+    private async Task Search()
+    {
+        _pageNumber = 1;
+        await FetchPage();
+    }
+
+    private async Task PrevPage() { _pageNumber--; await FetchPage(); }
+    private async Task NextPage() { _pageNumber++; await FetchPage(); }
+
+    private async Task FetchPage()
+    {
+        _searching = true;
+        _errorMessage = null;
+        try
+        {
+            var request = new ParkedMessageQueryRequest(
+                CorrelationId: Guid.NewGuid().ToString("N"),
+                SiteId: _selectedSiteId,
+                PageNumber: _pageNumber,
+                PageSize: _pageSize,
+                Timestamp: DateTimeOffset.UtcNow);
+
+            var response = await CommunicationService.QueryParkedMessagesAsync(_selectedSiteId, request);
+
+            if (response.Success)
+            {
+                _messages = response.Messages.ToList();
+                _totalCount = response.TotalCount;
+            }
+            else
+            {
+                _errorMessage = response.ErrorMessage ?? "Query failed.";
+            }
+        }
+        catch (Exception ex)
+        {
+            _errorMessage = $"Query failed: {ex.Message}";
+        }
+        _searching = false;
+    }
+}