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_90.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u067c\u0648\u0644 \u063a\u0648\u0631\u0647
 break A}if("keyboardToolbar.done"===e){y="\u0633\u0645 \u062f\u06cc"
 break A}y=null
 break A}return y},
-bqH(d){return new A.aVj(d)},
+bqH(d){return new A.aVi(d)},
 a_N:function a_N(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aRe:function aRe(){},
-aVj:function aVj(d){this.a=d},
+aRd:function aRd(){},
+aVi:function aVi(d){this.a=d},
 abW:function abW(){}}
 B=c[0]
 A=a.updateHolder(c[61],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aRe()}return x},
+x=y.z=new A.aRd()}return x},
 gbe(){return this.r}}
-A.aRe.prototype={
+A.aRd.prototype={
 gbj(){return"\u067e\u0631\u06d0 \u06a9\u0693\u0626"},
 gbi(){return"\u06a9\u0627\u067e\u064a \u06a9\u0693\u0626"},
 gbk(){return"\u067e\u06cc\u0633\u067c \u06a9\u0693\u0626"},
 gbg(){return"\u067c\u0648\u0644 \u063a\u0648\u0631\u0647 \u06a9\u0693\u0626"}}
 A.abW.prototype={}
 var z=a.updateTypes([])
-A.aVj.prototype={
+A.aVi.prototype={
 $1(d){return A.bpn(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.abW,B.Z)
 x(A.a_N,A.abW)
-x(A.aRe,B.b4)
-x(A.aVj,B.aW)
+x(A.aRd,B.b4)
+x(A.aVi,B.aW)
 y(A.abW,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a_N":{"Z":[]}}'))};
-(a=>{a["KxfXl/W7TwXIfvr3ApXFSbRTzYM="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["Podi9Zx1eB8wDJGsUKD3GgsPzjw="]=a.current})($__dart_deferred_initializers__);