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_56.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u0538\u0576\u057f\u0580\u0565\u056c \u0562
 break A}if("keyboardToolbar.done"===e){y="\u054a\u0561\u057f\u0580\u0561\u057d\u057f"
 break A}y=null
 break A}return y},
-br9(d){return new A.aVM(d)},
+br9(d){return new A.aVL(d)},
 a_m:function a_m(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQO:function aQO(){},
-aVM:function aVM(d){this.a=d},
+aQN:function aQN(){},
+aVL:function aVL(d){this.a=d},
 abv:function abv(){}}
 B=c[0]
 A=a.updateHolder(c[34],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQO()}return x},
+x=y.z=new A.aQN()}return x},
 gbe(){return this.r}}
-A.aQO.prototype={
+A.aQN.prototype={
 gbj(){return"\u053f\u057f\u0580\u0565\u056c"},
 gbi(){return"\u054a\u0561\u057f\u0573\u0565\u0576\u0565\u056c"},
 gbk(){return"\u054f\u0565\u0572\u0561\u0564\u0580\u0565\u056c"},
 gbg(){return"\u0538\u0576\u057f\u0580\u0565\u056c \u0562\u0578\u056c\u0578\u0580\u0568"}}
 A.abv.prototype={}
 var z=a.updateTypes([])
-A.aVM.prototype={
+A.aVL.prototype={
 $1(d){return A.bpQ(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abv,B.Z)
 x(A.a_m,A.abv)
-x(A.aQO,B.b4)
-x(A.aVM,B.aW)
+x(A.aQN,B.b4)
+x(A.aVL,B.aW)
 y(A.abv,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_m":{"Z":[]}}'))};
-(a=>{a["IhsYdylYu2lIMSZRU10tnaiGn94="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["UhKvyVVF0qqEQmnzB41O+Mii9+I="]=a.current})($__dart_deferred_initializers__);