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_5.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={
-b6o(d,e,f){var y,x,w=null,v=new B.a0(),u=new B.a0()
-u=new A.ZR(v,u)
+b6r(d,e,f){var y,x,w=null,v=new B.a0(),u=new B.a0()
+u=new A.ZT(v,u)
 u.bh(d,w,e,w)
 y=u.a
 x=v.r
 x.toString
 y.r=x
-v.r=A.brD(u)
+v.r=A.brG(u)
 return u},
-bqj(d,e){var y
+bqm(d,e){var y
 A:{if("timePicker.hours"===e){y="\u1230\u12d3\u1276\u127d"
 break A}if("timePicker.minutes"===e){y="\u12f0\u1242\u1243\u12ce\u127d"
 break A}if("timePicker.seconds"===e){y="\u1230\u12a8\u1295\u12f6\u127d"
@@ -23,38 +23,38 @@ break A}if("input.selectAll"===e){y="\u1201\u1209\u1295\u121d \u12ed\u121d\u1228
 break A}if("keyboardToolbar.done"===e){y="\u1270\u12a8\u1293\u12cd\u1297\u120d"
 break A}y=null
 break A}return y},
-brD(d){return new A.aWg(d)},
-ZR:function ZR(d,e){var _=this
+brG(d){return new A.aWi(d)},
+ZT:function ZT(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aQi:function aQi(){},
-aWg:function aWg(d){this.a=d},
-ab_:function ab_(){}}
+aQk:function aQk(){},
+aWi:function aWi(d){this.a=d},
+ab1:function ab1(){}}
 B=c[0]
 A=a.updateHolder(c[4],A)
-A.ZR.prototype={
+A.ZT.prototype={
 h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aQi()}return x},
+x=y.z=new A.aQk()}return x},
 gbe(){return this.r}}
-A.aQi.prototype={
+A.aQk.prototype={
 gbj(){return"\u1241\u1228\u1325"},
 gbi(){return"\u1245\u12f3"},
 gbk(){return"\u1208\u1325\u134d"},
 gbg(){return"\u1201\u1209\u1295\u121d \u12ed\u121d\u1228\u1321"}}
-A.ab_.prototype={}
+A.ab1.prototype={}
 var z=a.updateTypes([])
-A.aWg.prototype={
-$1(d){return A.bqj(this.a,d)},
+A.aWi.prototype={
+$1(d){return A.bqm(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
-x(A.ab_,B.Z)
-x(A.ZR,A.ab_)
-x(A.aQi,B.b4)
-x(A.aWg,B.aW)
-y(A.ab_,B.b_)})()
-B.b9(b.typeUniverse,JSON.parse('{"ZR":{"Z":[]}}'))};
-(a=>{a["j+bwBCQdu6tm9y+4m2IBa8wXLwI="]=a.current})($__dart_deferred_initializers__);
+x(A.ab1,B.Z)
+x(A.ZT,A.ab1)
+x(A.aQk,B.b4)
+x(A.aWi,B.aW)
+y(A.ab1,B.b_)})()
+B.b9(b.typeUniverse,JSON.parse('{"ZT":{"Z":[]}}'))};
+(a=>{a["Lpl2X3C5PRgTB+hpVxtW3pes0ec="]=a.current})($__dart_deferred_initializers__);