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.Host/Program.cs

@@ -20,9 +20,15 @@ using ScadaLink.StoreAndForward;
 using ScadaLink.TemplateEngine;
 using Serilog;
 
+// SCADALINK_CONFIG determines which role-specific config to load (Central or Site)
+// DOTNET_ENVIRONMENT/ASPNETCORE_ENVIRONMENT stay as "Development" for dev tooling (static assets, EF migrations, etc.)
+var scadalinkConfig = Environment.GetEnvironmentVariable("SCADALINK_CONFIG")
+    ?? Environment.GetEnvironmentVariable("DOTNET_ENVIRONMENT")
+    ?? "Production";
+
 var configuration = new ConfigurationBuilder()
     .AddJsonFile("appsettings.json", optional: false)
-    .AddJsonFile($"appsettings.{Environment.GetEnvironmentVariable("DOTNET_ENVIRONMENT") ?? "Production"}.json", optional: true)
+    .AddJsonFile($"appsettings.{scadalinkConfig}.json", optional: true)
     .AddEnvironmentVariables()
     .AddCommandLine(args)
     .Build();
@@ -110,6 +116,8 @@ try
 
         // Middleware pipeline
         app.UseStaticFiles();
+        app.UseWebSockets();
+        app.UseRouting();
         app.UseAuthentication();
         app.UseAuthorization();
         app.UseAntiforgery();
@@ -120,7 +128,7 @@ try
             ResponseWriter = UIResponseWriter.WriteHealthCheckUIResponse
         });
 
-        app.MapCentralUI();
+        app.MapCentralUI<ScadaLink.Host.Components.App>();
         app.MapInboundAPI();
         await app.RunAsync();
     }