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_79.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u0938\u0930\u094d\u0935 \u0928\u093f\u0935
 break A}if("keyboardToolbar.done"===e){y="\u092a\u0942\u0930\u094d\u0923"
 break A}y=null
 break A}return y},
-bqS(d){return new A.aVu(d)},
+bqS(d){return new A.aVt(d)},
 a_D:function a_D(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aR4:function aR4(){},
-aVu:function aVu(d){this.a=d},
+aR3:function aR3(){},
+aVt:function aVt(d){this.a=d},
 abM:function abM(){}}
 B=c[0]
 A=a.updateHolder(c[51],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aR4()}return x},
+x=y.z=new A.aR3()}return x},
 gbe(){return this.r}}
-A.aR4.prototype={
+A.aR3.prototype={
 gbj(){return"\u0915\u091f"},
 gbi(){return"\u0915\u0949\u092a\u0940"},
 gbk(){return"\u092a\u0947\u0938\u094d\u091f"},
 gbg(){return"\u0938\u0930\u094d\u0935 \u0928\u093f\u0935\u0921\u093e"}}
 A.abM.prototype={}
 var z=a.updateTypes([])
-A.aVu.prototype={
+A.aVt.prototype={
 $1(d){return A.bpy(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abM,B.Z)
 x(A.a_D,A.abM)
-x(A.aR4,B.b4)
-x(A.aVu,B.aW)
+x(A.aR3,B.b4)
+x(A.aVt,B.aW)
 y(A.abM,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_D":{"Z":[]}}'))};
-(a=>{a["1Pp60nCeIOmguJ/i5nMUMHjNTck="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["ERDQlzU1+3PlKZU5SEUNQfErXdU="]=a.current})($__dart_deferred_initializers__);