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_7.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u09b8\u0995\u09b2\u09cb \u09ac\u09be\u099b
 break A}if("keyboardToolbar.done"===e){y="\u09b8\u09ae\u09cd\u09aa\u09a8\u09cd\u09a8"
 break A}y=null
 break A}return y},
-brE(d){return new A.aWg(d)},
+brE(d){return new A.aWf(d)},
 ZV:function ZV(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQm:function aQm(){},
-aWg:function aWg(d){this.a=d},
+aQl:function aQl(){},
+aWf:function aWf(d){this.a=d},
 ab3:function ab3(){}}
 B=c[0]
 A=a.updateHolder(c[6],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQm()}return x},
+x=y.z=new A.aQl()}return x},
 gbe(){return this.r}}
-A.aQm.prototype={
+A.aQl.prototype={
 gbj(){return"\u0995\u09be\u099f"},
 gbi(){return"\u0995\u09aa\u09bf"},
 gbk(){return"\u09aa\u09c7\u09b7\u09cd\u099f"},
 gbg(){return"\u09b8\u0995\u09b2\u09cb \u09ac\u09be\u099b\u0995"}}
 A.ab3.prototype={}
 var z=a.updateTypes([])
-A.aWg.prototype={
+A.aWf.prototype={
 $1(d){return A.bqk(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.ab3,B.Z)
 x(A.ZV,A.ab3)
-x(A.aQm,B.b4)
-x(A.aWg,B.aW)
+x(A.aQl,B.b4)
+x(A.aWf,B.aW)
 y(A.ab3,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"ZV":{"Z":[]}}'))};
-(a=>{a["vV/5SV74+B+NV82Hj4uwBTRjYmo="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["ZT3ETSQ3E2t+71LjZpwLVqU7/wA="]=a.current})($__dart_deferred_initializers__);