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_1.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="Kies alles"
 break A}if("keyboardToolbar.done"===e){y="Klaar"
 break A}y=null
 break A}return y},
-brI(d){return new A.aWk(d)},
+brI(d){return new A.aWj(d)},
 ZS:function ZS(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQj:function aQj(){},
-aWk:function aWk(d){this.a=d},
+aQi:function aQi(){},
+aWj:function aWj(d){this.a=d},
 ab0:function ab0(){}}
 B=c[0]
 A=a.updateHolder(c[3],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQj()}return x},
+x=y.z=new A.aQi()}return x},
 gbe(){return this.r}}
-A.aQj.prototype={
+A.aQi.prototype={
 gbj(){return"Knip"},
 gbi(){return"Kopieer"},
 gbk(){return"Plak"},
 gbg(){return"Kies alles"}}
 A.ab0.prototype={}
 var z=a.updateTypes([])
-A.aWk.prototype={
+A.aWj.prototype={
 $1(d){return A.bqo(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.ab0,B.Z)
 x(A.ZS,A.ab0)
-x(A.aQj,B.b4)
-x(A.aWk,B.aW)
+x(A.aQi,B.b4)
+x(A.aWj,B.aW)
 y(A.ab0,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"ZS":{"Z":[]}}'))};
-(a=>{a["7Gsv5ww8LYE3p4lNvoHxt45dKCM="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["5Xy4YpRP2FY9eSNLXolltGuOO/8="]=a.current})($__dart_deferred_initializers__);