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/Properties/launchSettings.json

@@ -7,8 +7,9 @@
       "launchUrl": "",
       "applicationUrl": "https://localhost:5001;http://localhost:5000",
       "environmentVariables": {
-        "DOTNET_ENVIRONMENT": "Central",
-        "ASPNETCORE_ENVIRONMENT": "Development"
+        "DOTNET_ENVIRONMENT": "Development",
+        "ASPNETCORE_ENVIRONMENT": "Development",
+        "SCADALINK_CONFIG": "Central"
       }
     },
     "ScadaLink Site": {
@@ -16,8 +17,9 @@
       "dotnetRunMessages": true,
       "launchBrowser": false,
       "environmentVariables": {
-        "DOTNET_ENVIRONMENT": "Site",
-        "ASPNETCORE_ENVIRONMENT": "Development"
+        "DOTNET_ENVIRONMENT": "Development",
+        "ASPNETCORE_ENVIRONMENT": "Development",
+        "SCADALINK_CONFIG": "Site"
       }
     }
   }