feat: 集成离线数字货币图标系统

✨ 新功能：
- 下载16个主流币种图标（BTC, ETH, USDT, BNB, SOL等）
- 创建 CoinIcon 组件，支持离线图标和兜底显示
- 更新 CoinAvatar 组件，使用新的 CoinIcon
- 更新 market_page.dart 中的 _CoinAvatar 组件

📦 资源：
- 图标存放在 flutter_monisuo/assets/icons/crypto/
- 使用 Cryptocurrency Icons 开源图标库
- PNG 格式，128x128 像素

🎯 特性：
- 支持自定义大小
- 支持圆形背景
- 未找到图标时显示币种代码（兜底方案）
- 支持深色/浅色模式

📝 修改文件：
- 新增：flutter_monisuo/lib/ui/components/coin_icon.dart
- 新增：flutter_monisuo/assets/icons/crypto/*.png (16个)
- 修改：flutter_monisuo/pubspec.yaml
- 修改：flutter_monisuo/lib/ui/pages/trade/components/coin_avatar.dart
- 修改：flutter_monisuo/lib/ui/pages/market/market_page.dart

- ✅ 后端构建成功 (2.0s)
- ✅ Flutter Web 构建成功 (24.1s)
- ⚠️ Admin: TypeScript类型错误（已知问题）

flutter_monisuo/build/web/main.dart.js_78.part.js

@@ -1,15 +1,15 @@
 ((a,b)=>{a[b]=a[b]||{}})(self,"$__dart_deferred_initializers__")
 $__dart_deferred_initializers__.current=function(a,b,c,$){var B,A={
-b77(d,e,f){var y,x,w=null,v=new B.a0(),u=new B.a0()
-u=new A.a_A(v,u)
+b7a(d,e,f){var y,x,w=null,v=new B.a0(),u=new B.a0()
+u=new A.a_C(v,u)
 u.bh(d,w,e,w)
 y=u.a
 x=v.r
 x.toString
 y.r=x
-v.r=A.bqQ(u)
+v.r=A.bqT(u)
 return u},
-bpw(d,e){var y
+bpz(d,e){var y
 A:{if("timePicker.hours"===e){y="\u0426\u0430\u0433"
 break A}if("timePicker.minutes"===e){y="\u041c\u0438\u043d\u0443\u0442"
 break A}if("timePicker.seconds"===e){y="\u0421\u0435\u043a\u0443\u043d\u0434"
@@ -23,38 +23,38 @@ break A}if("input.selectAll"===e){y="\u0411\u04af\u0433\u0434\u0438\u0439\u0433
 break A}if("keyboardToolbar.done"===e){y="\u0411\u043e\u043b\u0441\u043e\u043d"
 break A}y=null
 break A}return y},
-bqQ(d){return new A.aVt(d)},
-a_A:function a_A(d,e){var _=this
+bqT(d){return new A.aVv(d)},
+a_C:function a_C(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aR1:function aR1(){},
-aVt:function aVt(d){this.a=d},
-abJ:function abJ(){}}
+aR3:function aR3(){},
+aVv:function aVv(d){this.a=d},
+abL:function abL(){}}
 B=c[0]
 A=a.updateHolder(c[50],A)
-A.a_A.prototype={
+A.a_C.prototype={
 h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aR1()}return x},
+x=y.z=new A.aR3()}return x},
 gbe(){return this.r}}
-A.aR1.prototype={
+A.aR3.prototype={
 gbj(){return"\u0422\u0430\u0439\u0440\u0430\u0445"},
 gbi(){return"\u0425\u0443\u0443\u043b\u0430\u0445"},
 gbk(){return"\u0411\u0443\u0443\u043b\u0433\u0430\u0445"},
 gbg(){return"\u0411\u04af\u0433\u0434\u0438\u0439\u0433 \u0441\u043e\u043d\u0433\u043e\u0445"}}
-A.abJ.prototype={}
+A.abL.prototype={}
 var z=a.updateTypes([])
-A.aVt.prototype={
-$1(d){return A.bpw(this.a,d)},
+A.aVv.prototype={
+$1(d){return A.bpz(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
-x(A.abJ,B.Z)
-x(A.a_A,A.abJ)
-x(A.aR1,B.b4)
-x(A.aVt,B.aW)
-y(A.abJ,B.b_)})()
-B.b9(b.typeUniverse,JSON.parse('{"a_A":{"Z":[]}}'))};
-(a=>{a["ftnx1Dr9LA/NSVnPTQpLYDgLYYQ="]=a.current})($__dart_deferred_initializers__);
+x(A.abL,B.Z)
+x(A.a_C,A.abL)
+x(A.aR3,B.b4)
+x(A.aVv,B.aW)
+y(A.abL,B.b_)})()
+B.b9(b.typeUniverse,JSON.parse('{"a_C":{"Z":[]}}'))};
+(a=>{a["qlGrMqU9KhnmVr6Xf7UQor5lq3o="]=a.current})($__dart_deferred_initializers__);