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_72.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u0411\u0430\u0430\u0440\u044b\u043d \u0442
 break A}if("keyboardToolbar.done"===e){y="\u0414\u0430\u044f\u0440"
 break A}y=null
 break A}return y},
-bqZ(d){return new A.aVB(d)},
+bqZ(d){return new A.aVA(d)},
 a_w:function a_w(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQY:function aQY(){},
-aVB:function aVB(d){this.a=d},
+aQX:function aQX(){},
+aVA:function aVA(d){this.a=d},
 abF:function abF(){}}
 B=c[0]
 A=a.updateHolder(c[44],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQY()}return x},
+x=y.z=new A.aQX()}return x},
 gbe(){return this.r}}
-A.aQY.prototype={
+A.aQX.prototype={
 gbj(){return"\u041a\u0435\u0441\u04af\u04af"},
 gbi(){return"\u041a\u04e9\u0447\u04af\u0440\u04af\u04af"},
 gbk(){return"\u0427\u0430\u043f\u0442\u043e\u043e"},
 gbg(){return"\u0411\u0430\u0430\u0440\u044b\u043d \u0442\u0430\u043d\u0434\u043e\u043e"}}
 A.abF.prototype={}
 var z=a.updateTypes([])
-A.aVB.prototype={
+A.aVA.prototype={
 $1(d){return A.bpF(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abF,B.Z)
 x(A.a_w,A.abF)
-x(A.aQY,B.b4)
-x(A.aVB,B.aW)
+x(A.aQX,B.b4)
+x(A.aVA,B.aW)
 y(A.abF,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_w":{"Z":[]}}'))};
-(a=>{a["EPHNiCBjpF4JsHrP+LC/2et4i1I="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["xN8I/UdfeDWIKxb8YUpuNLB03p0="]=a.current})($__dart_deferred_initializers__);