feat(auth): implement option 3 architecture - server InteractiveServer login + client WASM dashboard

- Add Server.AddInteractiveServerComponents() + AddInteractiveServerRenderMode()
- Create Server AuthLayout for login form (MudBlazor)
- Implement LoginSimple.razor with @rendermode InteractiveServer in Server project
- Update App.razor: CascadingAuthenticationState + Router with AppAssembly=Server
- Fix Client MudBlazor Providers in MainLayout + AuthLayout
- Update Playwright tests: use dynamic selectors for MudTextField (auto-generated IDs)
- Build: 0 errors, 0 warnings
- API: /api/auth/login fully operational (200 OK confirmed)
- Playwright E2E test: 1 passed

Architecture:
/login    → Server InteractiveServer (MudBlazor form)
/         → Client WebAssembly (Dashboard)
/api/*    → Server Endpoints

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/dotnet/QuantEngine.Web/Client/Layout/MainLayout.razor

@@ -1,8 +1,15 @@
 @inherits LayoutComponentBase
+@using QuantEngine.Web.Client.Theme
 @inject HttpClient Http
 @inject AuthenticationStateProvider AuthStateProvider
 @inject NavigationManager NavigationManager
 
+<!-- ✅ MudBlazor Providers (Required for Interactive WebAssembly) -->
+<MudThemeProvider Theme="@_theme" />
+<MudPopoverProvider />
+<MudDialogProvider />
+<MudSnackbarProvider />
+
 <MudLayout>
     <!-- Top Navigation Bar -->
     <MudAppBar Elevation="1" Dense="false" Color="Color.Surface" Class="mud-appbar-dense">
@@ -93,6 +100,7 @@
 </MudLayout>
 
 @code {
+    private MudTheme _theme = AppTheme.LightTheme;
     private bool navOpen = true;
     private bool fixedOpen = true;
     private string appVersion = "Local Debug";