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/pages/asset/components/action_buttons_row.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:lucide_icons_flutter/lucide_icons.dart';
+import '../../../../core/theme/app_spacing.dart';
 import '../../../../core/theme/app_theme.dart';
 
 /// 操作按钮行 — .pen node pIpHe
@@ -35,7 +36,7 @@ class ActionButtonsRow extends StatelessWidget {
           bgColor: bgColor,
           onTap: onDeposit,
         ),
-        const SizedBox(width: AppSpacing.sm + AppSpacing.xs),
+        const SizedBox(width: 12),
         ActionButton(
           icon: LucideIcons.arrowDownLeft,
           label: '提现',
@@ -43,7 +44,7 @@ class ActionButtonsRow extends StatelessWidget {
           bgColor: bgColor,
           onTap: onWithdraw,
         ),
-        const SizedBox(width: AppSpacing.sm + AppSpacing.xs),
+        const SizedBox(width: 12),
         ActionButton(
           icon: LucideIcons.repeat,
           label: '划转',
@@ -96,7 +97,7 @@ class ActionButton extends StatelessWidget {
                 color: accentColor,
               ),
             ),
-            const SizedBox(height: AppSpacing.xs + 2),
+            const SizedBox(height: 6),
             Text(
               label,
               style: AppTextStyles.labelLarge(context).copyWith(