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.Host/Components/App.razor

@@ -39,6 +39,7 @@
         });
     </script>
     <script src="/js/treeview-storage.js"></script>
+    <script src="_content/ScadaLink.CentralUI/js/monaco-init.js"></script>
     <script src="/lib/bootstrap/js/bootstrap.bundle.min.js"></script>
 </body>
 </html>