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_42.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u0627\u0646\u062a\u062e\u0627\u0628 \u0647
 break A}if("keyboardToolbar.done"===e){y="\u062a\u0645\u0627\u0645"
 break A}y=null
 break A}return y},
-brn(d){return new A.aW_(d)},
+brn(d){return new A.aVZ(d)},
 a_a:function a_a(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQC:function aQC(){},
-aW_:function aW_(d){this.a=d},
+aQB:function aQB(){},
+aVZ:function aVZ(d){this.a=d},
 abj:function abj(){}}
 B=c[0]
 A=a.updateHolder(c[22],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQC()}return x},
+x=y.z=new A.aQB()}return x},
 gbe(){return this.r}}
-A.aQC.prototype={
+A.aQB.prototype={
 gbj(){return"\u0628\u0631\u0634"},
 gbi(){return"\u06a9\u067e\u06cc"},
 gbk(){return"\u0686\u0633\u0628\u0627\u0646\u062f\u0646"},
 gbg(){return"\u0627\u0646\u062a\u062e\u0627\u0628 \u0647\u0645\u0647"}}
 A.abj.prototype={}
 var z=a.updateTypes([])
-A.aW_.prototype={
+A.aVZ.prototype={
 $1(d){return A.bq3(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abj,B.Z)
 x(A.a_a,A.abj)
-x(A.aQC,B.b4)
-x(A.aW_,B.aW)
+x(A.aQB,B.b4)
+x(A.aVZ,B.aW)
 y(A.abj,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_a":{"Z":[]}}'))};
-(a=>{a["y0ZTiupEXPgkWrfdXxAYcm3si0M="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["hyBDBp+xoZeNxPHIE2td+VSb8E4="]=a.current})($__dart_deferred_initializers__);