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/build/web/main.dart.js_63.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u3059\u3079\u3066\u9078\u629e"
 break A}if("keyboardToolbar.done"===e){y="\u5b8c\u4e86"
 break A}y=null
 break A}return y},
-br5(d){return new A.aVI(d)},
+br5(d){return new A.aVH(d)},
 a_q:function a_q(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQS:function aQS(){},
-aVI:function aVI(d){this.a=d},
+aQR:function aQR(){},
+aVH:function aVH(d){this.a=d},
 abz:function abz(){}}
 B=c[0]
 A=a.updateHolder(c[38],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQS()}return x},
+x=y.z=new A.aQR()}return x},
 gbe(){return this.r}}
-A.aQS.prototype={
+A.aQR.prototype={
 gbj(){return"\u30ab\u30c3\u30c8"},
 gbi(){return"\u30b3\u30d4\u30fc"},
 gbk(){return"\u30da\u30fc\u30b9\u30c8"},
 gbg(){return"\u3059\u3079\u3066\u9078\u629e"}}
 A.abz.prototype={}
 var z=a.updateTypes([])
-A.aVI.prototype={
+A.aVH.prototype={
 $1(d){return A.bpM(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abz,B.Z)
 x(A.a_q,A.abz)
-x(A.aQS,B.b4)
-x(A.aVI,B.aW)
+x(A.aQR,B.b4)
+x(A.aVH,B.aW)
 y(A.abz,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_q":{"Z":[]}}'))};
-(a=>{a["c6PE9X6oQXWBqN9HI5gpgR9EBkw="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["XfqFmtFYMxfKAj6qoykkxDCimno="]=a.current})($__dart_deferred_initializers__);