feat(ui/design): Monaco editor for script code fields

Vendors Monaco 0.55.1 min/vs/ (~15 MB) at
wwwroot/lib/monaco/vs/. No CDN dependency; works on air-gapped
deployments. Loaded lazily on first script-edit via the AMD loader.

wwwroot/js/monaco-init.js exposes window.MonacoBlazor with
createEditor / setValue / getValue / setMarkers / dispose. Handles
loader bootstrap, DotNet round-trip on content change, and marker
sets for later diagnostic wiring.

Components/Shared/MonacoEditor.razor is a Blazor wrapper with
Value / ValueChanged / Language / Height / ReadOnly parameters and
IAsyncDisposable teardown. Bidirectional binding tracks
_lastSentValue to avoid push/pull loops.

Replaces the plain textareas in SharedScriptForm, TemplateEdit's
Add-Script form, and ApiMethodForm. Default height 320px ≈ the
previous rows=10. Build / tests / dialog flow unaffected.

Wave 1 of three. Roslyn-backed completions and SCADA-specific
extensions follow in subsequent commits.

src/ScadaLink.CentralUI/Components/Pages/Design/ApiMethodForm.razor

@@ -38,7 +38,7 @@
                 </div>
                 <div class="mb-3">
                     <label class="form-label">Script</label>
-                    <textarea class="form-control font-monospace" rows="10" @bind="_script" style="font-size: 0.85rem;"></textarea>
+                    <MonacoEditor Value="@_script" ValueChanged="@(v => _script = v)" Language="csharp" Height="320px" />
                 </div>
 
                 @if (_formError != null)

src/ScadaLink.CentralUI/Components/Pages/Design/SharedScriptForm.razor

@@ -39,8 +39,7 @@
                 </div>
                 <div class="mb-2">
                     <label class="form-label small">Code</label>
-                    <textarea class="form-control form-control-sm font-monospace" rows="10" @bind="_formCode"
-                              style="font-size: 0.8rem;"></textarea>
+                    <MonacoEditor Value="@_formCode" ValueChanged="@(v => _formCode = v)" Language="csharp" Height="320px" />
                 </div>
                 @if (_formError != null)
                 {

src/ScadaLink.CentralUI/Components/Pages/Design/TemplateEdit.razor

@@ -654,8 +654,7 @@
                         </div>
                         <div class="col-12">
                             <label class="form-label">Code</label>
-                            <textarea class="form-control font-monospace" rows="10" @bind="_scriptCode"
-                                      style="font-size: 0.85rem;"></textarea>
+                            <MonacoEditor Value="@_scriptCode" ValueChanged="@(v => _scriptCode = v)" Language="csharp" Height="320px" />
                         </div>
                         @if (_scriptFormError != null)
                         {

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

@@ -0,0 +1,69 @@
+@namespace ScadaLink.CentralUI.Components.Shared
+@implements IAsyncDisposable
+@inject IJSRuntime JS
+
+<div @ref="_hostRef" class="monaco-editor-host"
+     style="height: @Height; border: 1px solid var(--bs-border-color); border-radius: 0.25rem; overflow: hidden;"></div>
+
+@code {
+    [Parameter] public string Value { get; set; } = "";
+    [Parameter] public EventCallback<string> ValueChanged { get; set; }
+    [Parameter] public string Language { get; set; } = "csharp";
+    [Parameter] public string Height { get; set; } = "320px";
+    [Parameter] public bool ReadOnly { get; set; } = false;
+
+    private ElementReference _hostRef;
+    private DotNetObjectReference<MonacoEditor>? _dotNetRef;
+    private readonly string _id = Guid.NewGuid().ToString("N");
+    private string _lastSentValue = "";
+    private bool _initialized;
+
+    protected override async Task OnAfterRenderAsync(bool firstRender)
+    {
+        if (firstRender)
+        {
+            _dotNetRef = DotNetObjectReference.Create(this);
+            _lastSentValue = Value ?? "";
+            try
+            {
+                await JS.InvokeVoidAsync(
+                    "MonacoBlazor.createEditor",
+                    _id,
+                    _hostRef,
+                    new
+                    {
+                        value = Value ?? "",
+                        language = Language,
+                        readOnly = ReadOnly
+                    },
+                    _dotNetRef);
+                _initialized = true;
+            }
+            catch
+            {
+                // Prerendering or JS not ready — swallow; subsequent render will retry.
+            }
+        }
+        else if (_initialized && (Value ?? "") != _lastSentValue)
+        {
+            _lastSentValue = Value ?? "";
+            try { await JS.InvokeVoidAsync("MonacoBlazor.setValue", _id, _lastSentValue); } catch { }
+        }
+    }
+
+    [JSInvokable]
+    public async Task OnValueChanged(string newValue)
+    {
+        _lastSentValue = newValue ?? "";
+        await ValueChanged.InvokeAsync(_lastSentValue);
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        if (_initialized)
+        {
+            try { await JS.InvokeVoidAsync("MonacoBlazor.dispose", _id); } catch { }
+        }
+        _dotNetRef?.Dispose();
+    }
+}