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/Components/Routes.razor

@@ -0,0 +1,31 @@
+<CascadingAuthenticationState>
+    <Router AppAssembly="typeof(Routes).Assembly"
+            AdditionalAssemblies="new[] { typeof(ScadaLink.CentralUI.Components.Layout.MainLayout).Assembly }">
+        <Found Context="routeData">
+            <AuthorizeRouteView RouteData="routeData" DefaultLayout="typeof(ScadaLink.CentralUI.Components.Layout.MainLayout)">
+                <NotAuthorized>
+                    @if (context.User.Identity?.IsAuthenticated != true)
+                    {
+                        <RedirectToLogin />
+                    }
+                    else
+                    {
+                        <NotAuthorizedView />
+                    }
+                </NotAuthorized>
+                <Authorizing>
+                    <p class="text-muted p-3">Checking authorization...</p>
+                </Authorizing>
+            </AuthorizeRouteView>
+        </Found>
+        <NotFound>
+            <LayoutView Layout="typeof(ScadaLink.CentralUI.Components.Layout.MainLayout)">
+                <div class="container mt-5">
+                    <h3>Page Not Found</h3>
+                    <p class="text-muted">The requested page does not exist.</p>
+                    <a href="/" class="btn btn-outline-primary btn-sm">Return to Dashboard</a>
+                </div>
+            </LayoutView>
+        </NotFound>
+    </Router>
+</CascadingAuthenticationState>