Add dark mode toggle with ThemeState service

Implement dark mode support using a new ThemeState service that manages theme switching and integrates with DevExpress Blazor theming. Update App.razor to apply the theme globally, enhance MainLayout with a toggle button and dynamic CSS classes, and add dark mode styles to CSS files. Register ThemeState as a scoped service in Program.cs.

DbFirst.BlazorWebApp/Program.cs

@@ -9,6 +9,7 @@ builder.Services.AddRazorComponents()
     .AddInteractiveServerComponents();
 
 builder.Services.AddDevExpressBlazor(options => options.BootstrapVersion = BootstrapVersion.v5);
+builder.Services.AddScoped<ThemeState>();
 
 var apiBaseUrl = builder.Configuration["ApiBaseUrl"];
 if (!string.IsNullOrWhiteSpace(apiBaseUrl))