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/ui/components/asset_card.dart

@@ -227,13 +227,13 @@ class AssetCardCompact extends StatelessWidget {
               Container(
                 padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 6),
                 decoration: BoxDecoration(
-                  color: getChangeBackgroundColor(isValueUp),
+                  color: AppColorScheme.getChangeBackgroundColor(isValueUp),
                   borderRadius: BorderRadius.circular(AppRadius.sm),
                 ),
                 child: Text(
                   change!,
                   style: TextStyle(
-                    color: getChangeColor(isValueUp),
+                    color: AppColorScheme.getChangeColor(isValueUp),
                     fontWeight: FontWeight.w600,
                   ),
                 ),

flutter_monisuo/lib/ui/components/coin_card.dart

@@ -81,13 +81,13 @@ class CoinCard extends StatelessWidget {
             Container(
               padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 6),
               decoration: BoxDecoration(
-                color: getChangeBackgroundColor(isUp),
+                color: AppColorScheme.getChangeBackgroundColor(isUp),
                 borderRadius: BorderRadius.circular(AppRadius.sm),
               ),
               child: Text(
                 change,
                 style: TextStyle(
-                  color: getChangeColor(isUp),
+                  color: AppColorScheme.getChangeColor(isUp),
                   fontWeight: FontWeight.w600,
                 ),
               ),