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_44.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="Piliin lahat"
 break A}if("keyboardToolbar.done"===e){y="Tapos"
 break A}y=null
 break A}return y},
-brk(d){return new A.aVX(d)},
+brk(d){return new A.aVW(d)},
 a_c:function a_c(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQE:function aQE(){},
-aVX:function aVX(d){this.a=d},
+aQD:function aQD(){},
+aVW:function aVW(d){this.a=d},
 abl:function abl(){}}
 B=c[0]
 A=a.updateHolder(c[24],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQE()}return x},
+x=y.z=new A.aQD()}return x},
 gbe(){return this.r}}
-A.aQE.prototype={
+A.aQD.prototype={
 gbj(){return"Gupitin"},
 gbi(){return"Kopyahin"},
 gbk(){return"I-paste"},
 gbg(){return"Piliin lahat"}}
 A.abl.prototype={}
 var z=a.updateTypes([])
-A.aVX.prototype={
+A.aVW.prototype={
 $1(d){return A.bq0(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abl,B.Z)
 x(A.a_c,A.abl)
-x(A.aQE,B.b4)
-x(A.aVX,B.aW)
+x(A.aQD,B.b4)
+x(A.aVW,B.aW)
 y(A.abl,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_c":{"Z":[]}}'))};
-(a=>{a["jArT/e07CBq2YrnMcv+h6/WrOI8="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["60EeWfweQHQpkGXFXDiFwB4Qv60="]=a.current})($__dart_deferred_initializers__);