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_76.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u0418\u0437\u0431\u0435\u0440\u0438 \u0441
 break A}if("keyboardToolbar.done"===e){y="\u0413\u043e\u0442\u043e\u0432\u043e"
 break A}y=null
 break A}return y},
-bqV(d){return new A.aVx(d)},
+bqV(d){return new A.aVw(d)},
 a_A:function a_A(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aR1:function aR1(){},
-aVx:function aVx(d){this.a=d},
+aR0:function aR0(){},
+aVw:function aVw(d){this.a=d},
 abJ:function abJ(){}}
 B=c[0]
 A=a.updateHolder(c[48],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aR1()}return x},
+x=y.z=new A.aR0()}return x},
 gbe(){return this.r}}
-A.aR1.prototype={
+A.aR0.prototype={
 gbj(){return"\u0418\u0441\u0435\u0447\u0438"},
 gbi(){return"\u041a\u043e\u043f\u0438\u0440\u0430\u0458"},
 gbk(){return"\u0417\u0430\u043b\u0435\u043f\u0438"},
 gbg(){return"\u0418\u0437\u0431\u0435\u0440\u0438 \u0441\xe8"}}
 A.abJ.prototype={}
 var z=a.updateTypes([])
-A.aVx.prototype={
+A.aVw.prototype={
 $1(d){return A.bpB(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abJ,B.Z)
 x(A.a_A,A.abJ)
-x(A.aR1,B.b4)
-x(A.aVx,B.aW)
+x(A.aR0,B.b4)
+x(A.aVw,B.aW)
 y(A.abJ,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_A":{"Z":[]}}'))};
-(a=>{a["3omQp+O9i/ceMKo4qmav2zFfXJI="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["irB3oVcgGbI9+Gjee9CbMbtE6xw="]=a.current})($__dart_deferred_initializers__);