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_18.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u09b8\u09ac \u09a8\u09bf\u09b0\u09cd\u09ac
 break A}if("keyboardToolbar.done"===e){y="\u09b8\u09ae\u09cd\u09aa\u09a8\u09cd\u09a8"
 break A}y=null
 break A}return y},
-brA(d){return new A.aWc(d)},
+brA(d){return new A.aWb(d)},
 ZZ:function ZZ(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQq:function aQq(){},
-aWc:function aWc(d){this.a=d},
+aQp:function aQp(){},
+aWb:function aWb(d){this.a=d},
 ab7:function ab7(){}}
 B=c[0]
 A=a.updateHolder(c[10],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){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"\u0995\u09be\u099f"},
 gbi(){return"\u0995\u09aa\u09bf"},
 gbk(){return"\u09aa\u09c7\u09b8\u09cd\u099f"},
 gbg(){return"\u09b8\u09ac \u09a8\u09bf\u09b0\u09cd\u09ac\u09be\u099a\u09a8 \u0995\u09b0\u09c1\u09a8"}}
 A.ab7.prototype={}
 var z=a.updateTypes([])
-A.aWc.prototype={
+A.aWb.prototype={
 $1(d){return A.bqg(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.ab7,B.Z)
 x(A.ZZ,A.ab7)
-x(A.aQq,B.b4)
-x(A.aWc,B.aW)
+x(A.aQp,B.b4)
+x(A.aWb,B.aW)
 y(A.ab7,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"ZZ":{"Z":[]}}'))};
-(a=>{a["Ac4llgfUFbu0JhnPSx3NvjdN49s="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["Os6fXDCWD8vUlt+y86uhacUECCU="]=a.current})($__dart_deferred_initializers__);