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/_Imports.razor

@@ -4,6 +4,7 @@
 @using Microsoft.AspNetCore.Components.Forms
 @using Microsoft.AspNetCore.Components.Routing
 @using Microsoft.AspNetCore.Components.Web
+@using static Microsoft.AspNetCore.Components.Web.RenderMode
 @using ScadaLink.CentralUI
 @using ScadaLink.CentralUI.Components.Layout
 @using ScadaLink.CentralUI.Components.Shared