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/core/theme/app_color_scheme.dart

@@ -426,14 +426,14 @@ class AppColorScheme {
 
   /// 获取买入按钮渐变（主题感知）- 用于盈利展示
 
-/// 获取涨跌颜色（明暗通用）
-Color getChangeColor(bool isUp) => isUp ? AppColorScheme.up : AppColorScheme.down;
+  /// 获取涨跌颜色（明暗通用）
+  static Color getChangeColor(bool isUp) => isUp ? AppColorScheme.up : AppColorScheme.down;
 
-/// 获取涨跌背景色（带透明度）
-Color getChangeBackgroundColor(bool isUp, {double opacity = 0.15}) {
-  return isUp
-    ? AppColorScheme.up.withValues(alpha: opacity)
-    : AppColorScheme.down.withValues(alpha: opacity);
-}
+  /// 获取涨跌背景色（带透明度）
+  static Color getChangeBackgroundColor(bool isUp, {double opacity = 0.15}) {
+    return isUp
+        ? AppColorScheme.up.withValues(alpha: opacity)
+        : AppColorScheme.down.withValues(alpha: opacity);
+  }
 }