fix(theme): 修复组件主题硬编码问题

- asset_card: Colors.white/black → colorScheme
- gradient_button: Colors.white → onPrimary
- trade_button: Colors.white → onPrimary
- 支持亮/暗主题动态切换

flutter_monisuo/lib/ui/components/gradient_button.dart

@@ -96,6 +96,9 @@ class GradientButton extends StatelessWidget {
     final buttonGradient = gradient ??
         (isDark ? AppColorScheme.darkCtaGradient : AppColorScheme.lightCtaGradient);
 
+    // 主题感知颜色 - 在渐变背景上使用 onPrimary
+    final textColor = colorScheme.onPrimary;
+
     return Container(
       width: isFullWidth ? double.infinity : null,
       height: height,
@@ -114,23 +117,23 @@ class GradientButton extends StatelessWidget {
               mainAxisAlignment: MainAxisAlignment.center,
               children: [
                 if (isLoading)
-                  const SizedBox(
+                  SizedBox(
                     width: 20,
                     height: 20,
                     child: CircularProgressIndicator(
                       strokeWidth: 2,
-                      color: Colors.white,
+                      color: textColor,
                     ),
                   )
                 else ...[
                   if (icon != null) ...[
-                    Icon(icon, size: 18, color: Colors.white),
+                    Icon(icon, size: 18, color: textColor),
                     SizedBox(width: AppSpacing.sm),
                   ],
                   Text(
                     text,
-                    style: const TextStyle(
-                      color: Colors.white,
+                    style: TextStyle(
+                      color: textColor,
                       fontSize: 16,
                       fontWeight: FontWeight.w600,
                     ),