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_107.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u06be\u06d5\u0645\u0645\u0649\u0646\u0649
 break A}if("keyboardToolbar.done"===e){y="\u062a\u0627\u0645\u0627\u0645"
 break A}y=null
 break A}return y},
-brl(d){return new A.aVY(d)},
+brl(d){return new A.aVX(d)},
 a02:function a02(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aRu:function aRu(){},
-aVY:function aVY(d){this.a=d},
+aRt:function aRt(){},
+aVX:function aVX(d){this.a=d},
 acb:function acb(){}}
 B=c[0]
 A=a.updateHolder(c[77],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aRu()}return x},
+x=y.z=new A.aRt()}return x},
 gbe(){return this.r}}
-A.aRu.prototype={
+A.aRt.prototype={
 gbj(){return"\u0643\u06d5\u0633"},
 gbi(){return"\u0643\u06c6\u0686\u06c8\u0631"},
 gbk(){return"\u0686\u0627\u067e\u0644\u0627"},
 gbg(){return"\u06be\u06d5\u0645\u0645\u0649\u0646\u0649 \u062a\u0627\u0644\u0644\u0627"}}
 A.acb.prototype={}
 var z=a.updateTypes([])
-A.aVY.prototype={
+A.aVX.prototype={
 $1(d){return A.bq1(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.acb,B.Z)
 x(A.a02,A.acb)
-x(A.aRu,B.b4)
-x(A.aVY,B.aW)
+x(A.aRt,B.b4)
+x(A.aVX,B.aW)
 y(A.acb,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a02":{"Z":[]}}'))};
-(a=>{a["gQYSc7P/dCk6RMCJr91QEz8oedY="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["j+s4exqRwwjj73Z0QE/zweeIuDM="]=a.current})($__dart_deferred_initializers__);