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

src/dotnet/QuantEngine.Web/Program.cs

@@ -157,7 +157,7 @@ app.UseStaticFiles(new StaticFileOptions
 app.UseStatusCodePages(async ctx =>
 {
     var path = ctx.HttpContext.Request.Path.Value ?? "";
-    if (!path.StartsWithSegments("/api", StringComparison.OrdinalIgnoreCase)
+    if (!path.StartsWith("/api", StringComparison.OrdinalIgnoreCase)
         && !path.StartsWith("/Account/", StringComparison.OrdinalIgnoreCase))
     {
         ctx.HttpContext.Response.Redirect("/not-found");
@@ -194,6 +194,12 @@ app.MapGet("/", async (HttpContext ctx) =>
     await Task.CompletedTask;
 });
 
+// Redirect /login to /login.html (static file)
+app.MapGet("/login", (HttpContext ctx) =>
+{
+    ctx.Response.Redirect("/login.html", permanent: false);
+});
+
 // Collection API Endpoints (must be before MapRazorComponents)
 app.MapCollectionEndpoints();