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_33.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="Alles ausw\xe4hlen"
 break A}if("keyboardToolbar.done"===e){y="Fertig"
 break A}y=null
 break A}return y},
-brs(d){return new A.aW4(d)},
+brs(d){return new A.aW3(d)},
 a_5:function a_5(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQx:function aQx(){},
-aW4:function aW4(d){this.a=d},
+aQw:function aQw(){},
+aW3:function aW3(d){this.a=d},
 abe:function abe(){}}
 B=c[0]
 A=a.updateHolder(c[17],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQx()}return x},
+x=y.z=new A.aQw()}return x},
 gbe(){return this.r}}
-A.aQx.prototype={
+A.aQw.prototype={
 gbj(){return"Ausschneiden"},
 gbi(){return"Kopieren"},
 gbk(){return"Einf\xfcgen"},
 gbg(){return"Alles ausw\xe4hlen"}}
 A.abe.prototype={}
 var z=a.updateTypes([])
-A.aW4.prototype={
+A.aW3.prototype={
 $1(d){return A.bq8(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abe,B.Z)
 x(A.a_5,A.abe)
-x(A.aQx,B.b4)
-x(A.aW4,B.aW)
+x(A.aQw,B.b4)
+x(A.aW3,B.aW)
 y(A.abe,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_5":{"Z":[]}}'))};
-(a=>{a["dyH6Sc+yCePvMfO1eL0XuedJUUQ="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["cmjL5mTqas9v1bFQRehgGJNsagE="]=a.current})($__dart_deferred_initializers__);