Fix blazor.web.js 404: move App.razor and Routes.razor to Host project

Root cause: App.razor was in CentralUI (Microsoft.NET.Sdk.Razor RCL) but
MapRazorComponents<App>().AddInteractiveServerRenderMode() serves
_framework/blazor.web.js from the host assembly (Microsoft.NET.Sdk.Web).
Per MS docs, the root component must be in the server (host) project.

- Move App.razor and Routes.razor from CentralUI to Host/Components/
- Add Host/_Imports.razor for Razor component usings
- Make MapCentralUI generic: MapCentralUI<TApp>() accepts root component
- Add AdditionalAssemblies to discover CentralUI's pages/layouts
- Routes.razor references both Host and CentralUI assemblies
- Fix all DI registrations (TemplateEngine services)
- SCADALINK_CONFIG env var for role-specific config loading

Verified: blazor.web.js HTTP 200 (200KB), login page renders with Blazor
interactive server mode, SignalR circuit establishes, LDAP auth works.

src/ScadaLink.CentralUI/EndpointExtensions.cs

@@ -1,18 +1,24 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Routing;
 using ScadaLink.CentralUI.Auth;
-using ScadaLink.CentralUI.Components;
+using ScadaLink.CentralUI.Components.Layout;
 
 namespace ScadaLink.CentralUI;
 
 public static class EndpointExtensions
 {
-    public static IEndpointRouteBuilder MapCentralUI(this IEndpointRouteBuilder endpoints)
+    /// <summary>
+    /// Maps the Central UI endpoints. The caller must provide the root App component type
+    /// from the Host assembly so that blazor.web.js is served correctly.
+    /// </summary>
+    public static IEndpointRouteBuilder MapCentralUI<TApp>(this IEndpointRouteBuilder endpoints)
+        where TApp : Microsoft.AspNetCore.Components.IComponent
     {
         endpoints.MapAuthEndpoints();
 
-        endpoints.MapRazorComponents<App>()
-            .AddInteractiveServerRenderMode();
+        endpoints.MapRazorComponents<TApp>()
+            .AddInteractiveServerRenderMode()
+            .AddAdditionalAssemblies(typeof(MainLayout).Assembly);
 
         return endpoints;
     }