feat: 首页和资产页面使用 CoinIcon 显示币种图标

✨ 修改内容：
- 首页 (home_page.dart): 持仓列表使用 CoinIcon 替换 CircleAvatar
- 资产持仓 (holdings_section.dart): 使用 CoinIcon 替换文字显示

🎨 效果：
- 首页：持仓卡片显示真实的币种图标
- 资产页面：持仓列表显示真实的币种图标
- 未找到图标时自动显示币种代码（兜底方案）

- ✅ Flutter Web 构建成功 (23.7s)

flutter_monisuo/lib/ui/pages/asset/components/holdings_section.dart

@@ -4,6 +4,7 @@ import '../../../../core/theme/app_theme_extension.dart';
 import '../../../../core/theme/app_spacing.dart';
 import '../../../../data/models/account_models.dart';
 import '../../../components/glass_panel.dart';
+import '../../../components/coin_icon.dart';
 
 /// 持倉區域
 /// Header: "我的資產" + "查看全部 >"
@@ -128,17 +129,10 @@ class HoldingRow extends StatelessWidget {
           Container(
             width: 36,
             height: 36,
-            decoration: BoxDecoration(
-              color: accentBgColor,
-              borderRadius: BorderRadius.circular(18),
-            ),
-            alignment: Alignment.center,
-            child: Text(
-              coinCode.substring(0, 1),
-              style: AppTextStyles.headlineMedium(context).copyWith(
-                fontWeight: FontWeight.w700,
-                color: accentColor,
-              ),
+            child: CoinIcon(
+              symbol: coinCode,
+              size: 36,
+              isCircle: false,
             ),
           ),
           const SizedBox(width: 10),

flutter_monisuo/lib/ui/pages/home/home_page.dart

@@ -14,6 +14,7 @@ import '../../components/glass_panel.dart';
 import '../mine/welfare_center_page.dart';
 import '../asset/transfer_page.dart';
 import '../asset/components/asset_dialogs.dart';
+import '../../components/coin_icon.dart';
 import 'header_bar.dart';
 import 'quick_actions_row.dart';
 import 'hot_coins_section.dart';
@@ -569,16 +570,10 @@ class _HoldingItem extends StatelessWidget {
         children: [
           Row(
             children: [
-              CircleAvatar(
-                radius: 18,
-                backgroundColor: context.colors.primary.withValues(alpha: 0.1),
-                child: Text(
-                  holding.coinCode.substring(0, 1),
-                  style: AppTextStyles.headlineMedium(context).copyWith(
-                    color: context.colors.primary,
-                    fontWeight: FontWeight.bold,
-                  ),
-                ),
+              CoinIcon(
+                symbol: holding.coinCode,
+                size: 36,
+                isCircle: false,
               ),
               SizedBox(width: AppSpacing.sm + AppSpacing.xs),
               Column(