fix: resolve login page CSS styling issues

- Map /login route to static login.html file to serve embedded CSS correctly
- Redirect /login to /login.html for proper static file delivery
- Fix NavigationContext namespace ambiguity in App.razor (MudBlazor vs ASP.NET)
- Fix StatusCodePages middleware path validation (StartsWithSegments → StartsWith)

Login page now displays with:
- Gradient background with frosted glass card design
- Properly styled form inputs and validation
- Professional Material Design appearance
- Working client-side authentication flow

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

src/dotnet/QuantEngine.Web/Components/App.razor

@@ -35,7 +35,7 @@
 </html>
 
 @code {
-    private async Task OnNavigateAsync(NavigationContext context)
+    private async Task OnNavigateAsync(Microsoft.AspNetCore.Components.Routing.NavigationContext context)
     {
         // /Account/* paths are Razor Pages, not Blazor components
         // Force browser navigation instead of Blazor routing