feat(theme): migrate to new theme system with static color methods

- Convert instance methods to static methods in AppColorScheme for
  getChangeColor and getChangeBackgroundColor
- Update main.dart to use ShadThemeData with AppColorScheme color
  schemes instead of createLight/DarkShadTheme functions
- Add app_theme.dart import to main.dart
- Refactor asset_card.dart and coin_card.dart to call static methods
  via AppColorScheme class
- Add app_spacing.dart import to action_buttons_row.dart
- Replace SizedBox with proper spacing constants in action buttons row

flutter_monisuo/lib/main.dart

@@ -10,6 +10,7 @@ import 'package:shared_preferences/shared_preferences.dart';
 import 'core/network/dio_client.dart';
 import 'core/storage/local_storage.dart';
 import 'core/theme/app_color_scheme.dart';
+import 'core/theme/app_theme.dart';
 import 'core/event/app_event_bus.dart';
 import 'data/services/user_service.dart';
 import 'data/services/market_service.dart';
@@ -68,8 +69,14 @@ class MyApp extends StatelessWidget {
         builder: (context, themeProvider, _) {
           return ShadApp.custom(
             themeMode: themeProvider.themeMode,
-            theme: createLightShadTheme(),
-            darkTheme: createDarkShadTheme(),
+            theme: ShadThemeData(
+              colorScheme: AppColorScheme.lightShad,
+              brightness: Brightness.light,
+            ),
+            darkTheme: ShadThemeData(
+              colorScheme: AppColorScheme.darkShad,
+              brightness: Brightness.dark,
+            ),
             appBuilder: _buildMaterialApp,
           );
         },