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_78.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u0411\u04af\u0433\u0434\u0438\u0439\u0433
 break A}if("keyboardToolbar.done"===e){y="\u0411\u043e\u043b\u0441\u043e\u043d"
 break A}y=null
 break A}return y},
-bqT(d){return new A.aVv(d)},
+bqT(d){return new A.aVu(d)},
 a_C:function a_C(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aR3:function aR3(){},
-aVv:function aVv(d){this.a=d},
+aR2:function aR2(){},
+aVu:function aVu(d){this.a=d},
 abL:function abL(){}}
 B=c[0]
 A=a.updateHolder(c[50],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aR3()}return x},
+x=y.z=new A.aR2()}return x},
 gbe(){return this.r}}
-A.aR3.prototype={
+A.aR2.prototype={
 gbj(){return"\u0422\u0430\u0439\u0440\u0430\u0445"},
 gbi(){return"\u0425\u0443\u0443\u043b\u0430\u0445"},
 gbk(){return"\u0411\u0443\u0443\u043b\u0433\u0430\u0445"},
 gbg(){return"\u0411\u04af\u0433\u0434\u0438\u0439\u0433 \u0441\u043e\u043d\u0433\u043e\u0445"}}
 A.abL.prototype={}
 var z=a.updateTypes([])
-A.aVv.prototype={
+A.aVu.prototype={
 $1(d){return A.bpz(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abL,B.Z)
 x(A.a_C,A.abL)
-x(A.aR3,B.b4)
-x(A.aVv,B.aW)
+x(A.aR2,B.b4)
+x(A.aVu,B.aW)
 y(A.abL,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_C":{"Z":[]}}'))};
-(a=>{a["qlGrMqU9KhnmVr6Xf7UQor5lq3o="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["cu+VQtvoCnRiRY2IeQEvYAgGN9I="]=a.current})($__dart_deferred_initializers__);