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/Components/App.razor

@@ -5,7 +5,10 @@
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <base href="/" />
-    @DxResourceManager.RegisterTheme(Themes.Fluent)
+    @DxResourceManager.RegisterTheme(Themes.Fluent.Clone(properties =>
+    {
+        properties.ApplyToPageElements = true;
+    }))
     @DxResourceManager.RegisterScripts()
 
     <link href="_content/DevExpress.Blazor.Dashboard/ace.css" rel="stylesheet" />