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_13.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u0418\u0437\u0431\u0435\u0440\u0438 \u0432
 break A}if("keyboardToolbar.done"===e){y="\u0413\u043e\u0442\u043e\u0432\u043e"
 break A}y=null
 break A}return y},
-brB(d){return new A.aWd(d)},
+brB(d){return new A.aWc(d)},
 ZY:function ZY(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQp:function aQp(){},
-aWd:function aWd(d){this.a=d},
+aQo:function aQo(){},
+aWc:function aWc(d){this.a=d},
 ab6:function ab6(){}}
 B=c[0]
 A=a.updateHolder(c[9],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQp()}return x},
+x=y.z=new A.aQo()}return x},
 gbe(){return this.r}}
-A.aQp.prototype={
+A.aQo.prototype={
 gbj(){return"\u0418\u0437\u0440\u0435\u0436\u0438"},
 gbi(){return"\u041a\u043e\u043f\u0438\u0440\u0430\u0439"},
 gbk(){return"\u041f\u043e\u0441\u0442\u0430\u0432\u0438"},
 gbg(){return"\u0418\u0437\u0431\u0435\u0440\u0438 \u0432\u0441\u0438\u0447\u043a\u043e"}}
 A.ab6.prototype={}
 var z=a.updateTypes([])
-A.aWd.prototype={
+A.aWc.prototype={
 $1(d){return A.bqh(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.ab6,B.Z)
 x(A.ZY,A.ab6)
-x(A.aQp,B.b4)
-x(A.aWd,B.aW)
+x(A.aQo,B.b4)
+x(A.aWc,B.aW)
 y(A.ab6,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"ZY":{"Z":[]}}'))};
-(a=>{a["83nQENMkyLCGxA/5/VCDavweUh0="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["QmkCGVvnAbF582YEcnAvJdou3dU="]=a.current})($__dart_deferred_initializers__);