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/trade_button.dart

@@ -45,11 +45,15 @@ class TradeButton extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final colorScheme = Theme.of(context).colorScheme;
     final gradient = isBuy ? AppColorScheme.buyGradient : AppColorScheme.sellGradient;
     final text = isBuy
         ? '买入${coinCode != null ? ' $coinCode' : ''}'
         : '卖出${coinCode != null ? ' $coinCode' : ''}';
 
+    // 主题感知颜色 - 在渐变背景上使用 onPrimary
+    final textColor = colorScheme.onPrimary;
+
     return Container(
       width: fullWidth ? double.infinity : null,
       height: 48,
@@ -68,24 +72,24 @@ class TradeButton extends StatelessWidget {
               mainAxisAlignment: MainAxisAlignment.center,
               children: [
                 if (isLoading)
-                  const SizedBox.square(
+                  SizedBox.square(
                     dimension: 16,
                     child: CircularProgressIndicator(
                       strokeWidth: 2,
-                      color: Colors.white,
+                      color: textColor,
                     ),
                   )
                 else
                   Icon(
                     isBuy ? Icons.arrow_downward_rounded : Icons.arrow_upward_rounded,
                     size: 18,
-                    color: Colors.white,
+                    color: textColor,
                   ),
                 const SizedBox(width: 8),
                 Text(
                   text,
-                  style: const TextStyle(
-                    color: Colors.white,
+                  style: TextStyle(
+                    color: textColor,
                     fontSize: 16,
                     fontWeight: FontWeight.w600,
                   ),