feat(ui): 新增精選卡片漸變色並優化市場頁面設計

flutter_monisuo/lib/core/theme/app_color_scheme.dart

@@ -133,6 +133,30 @@ class AppColorScheme {
   static const Color lightOnSurfaceVariant = Color(0xFF475569);
   static const Color lightOnSurfaceMuted = Color(0xFF94A3B8);
 
+  // ============================================
+  // 精選卡片漸變色 (Featured Card Gradients)
+  // ============================================
+
+  /// BTC 卡片漸變 - 深藍色 (#1A1A2E → #0F3460)
+  static const LinearGradient btcCardGradient = LinearGradient(
+    colors: [Color(0xFF1A1A2E), Color(0xFF0F3460)],
+    begin: Alignment.topLeft,
+    end: Alignment.bottomRight,
+  );
+
+  /// BTC 柱狀圖顏色 - 金色
+  static const Color btcBarColor = Color(0xFFD4AF37);
+
+  /// ETH 卡片漸變 - 深紫色 (#1A1040 → #312E81)
+  static const LinearGradient ethCardGradient = LinearGradient(
+    colors: [Color(0xFF1A1040), Color(0xFF312E81)],
+    begin: Alignment.topLeft,
+    end: Alignment.bottomRight,
+  );
+
+  /// ETH 柱狀圖顏色 - 紫色
+  static const Color ethBarColor = Color(0xFF818CF8);
+
   // ============================================
   // Glass Panel 毛玻璃效果顏色
   // ============================================