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_59.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="Velja allt"
 break A}if("keyboardToolbar.done"===e){y="Loki\xf0"
 break A}y=null
 break A}return y},
-br7(d){return new A.aVK(d)},
+br7(d){return new A.aVJ(d)},
 a_o:function a_o(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQQ:function aQQ(){},
-aVK:function aVK(d){this.a=d},
+aQP:function aQP(){},
+aVJ:function aVJ(d){this.a=d},
 abx:function abx(){}}
 B=c[0]
 A=a.updateHolder(c[36],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQQ()}return x},
+x=y.z=new A.aQP()}return x},
 gbe(){return this.r}}
-A.aQQ.prototype={
+A.aQP.prototype={
 gbj(){return"Klippa"},
 gbi(){return"Afrita"},
 gbk(){return"L\xedma"},
 gbg(){return"Velja allt"}}
 A.abx.prototype={}
 var z=a.updateTypes([])
-A.aVK.prototype={
+A.aVJ.prototype={
 $1(d){return A.bpO(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abx,B.Z)
 x(A.a_o,A.abx)
-x(A.aQQ,B.b4)
-x(A.aVK,B.aW)
+x(A.aQP,B.b4)
+x(A.aVJ,B.aW)
 y(A.abx,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_o":{"Z":[]}}'))};
-(a=>{a["YRcfzj4ZB4Hjb3gd1401C5d8x58="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["7mgWbTKXPmj2CuoGZSf5G1nsbfw="]=a.current})($__dart_deferred_initializers__);