Refactor dark mode color blending to extension method

Moved dark mode color blending logic to a Color extension (toDark) in the design system. Updated usages in ui_utils.dart and related tests to use the new extension. Removed the old getDarkModeColor function for cleaner and more idiomatic code organization.

packages/apidash_design_system/lib/extensions/color_extensions.dart

@@ -0,0 +1,9 @@
+import 'package:flutter/material.dart';
+import '../tokens/colors.dart';
+
+extension ColorExtension on Color {
+  Color get toDark => Color.alphaBlend(
+        withValues(alpha: kOpacityDarkModeBlend),
+        kColorWhite,
+      );
+}

packages/apidash_design_system/lib/extensions/extensions.dart

@@ -1 +1,2 @@
+export 'color_extensions.dart';
 export 'context_extensions.dart';