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_88.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u0a38\u0a2d \u0a1a\u0a41\u0a23\u0a4b"
 break A}if("keyboardToolbar.done"===e){y="\u0a39\u0a4b \u0a17\u0a3f\u0a06"
 break A}y=null
 break A}return y},
-bqJ(d){return new A.aVl(d)},
+bqJ(d){return new A.aVk(d)},
 a_L:function a_L(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aRc:function aRc(){},
-aVl:function aVl(d){this.a=d},
+aRb:function aRb(){},
+aVk:function aVk(d){this.a=d},
 abU:function abU(){}}
 B=c[0]
 A=a.updateHolder(c[59],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aRc()}return x},
+x=y.z=new A.aRb()}return x},
 gbe(){return this.r}}
-A.aRc.prototype={
+A.aRb.prototype={
 gbj(){return"\u0a15\u0a71\u0a1f\u0a4b"},
 gbi(){return"\u0a15\u0a3e\u0a2a\u0a40 \u0a15\u0a30\u0a4b"},
 gbk(){return"\u0a2a\u0a47\u0a38\u0a1f \u0a15\u0a30\u0a4b"},
 gbg(){return"\u0a38\u0a2d \u0a1a\u0a41\u0a23\u0a4b"}}
 A.abU.prototype={}
 var z=a.updateTypes([])
-A.aVl.prototype={
+A.aVk.prototype={
 $1(d){return A.bpp(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abU,B.Z)
 x(A.a_L,A.abU)
-x(A.aRc,B.b4)
-x(A.aVl,B.aW)
+x(A.aRb,B.b4)
+x(A.aVk,B.aW)
 y(A.abU,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_L":{"Z":[]}}'))};
-(a=>{a["kNofFY4fii89KVGZGxKm6dbQhyA="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["iIyul8APTJG5m7i5gvLeywPoNck="]=a.current})($__dart_deferred_initializers__);