feat(client): 商品详情页支持拍照添加照片（Android/iOS）

- 移动端加图改用 image_picker，底部弹出「拍照 / 从相册选择」二选一；
  桌面/Web 保持 file_picker 文件选择
- 用 defaultTargetPlatform 判断移动端，避免在参与 Web 编译的文件引入 dart:io
- iOS：Info.plist 增加 NSCameraUsageDescription / NSPhotoLibraryUsageDescription
- Android：拍照走系统相机、相册走 Photo Picker，均无需声明权限

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/macos/Flutter/GeneratedPluginRegistrant.swift

@@ -6,6 +6,7 @@ import FlutterMacOS
 import Foundation
 
 import file_picker
+import file_selector_macos
 import package_info_plus
 import printing
 import shared_preferences_foundation
@@ -13,6 +14,7 @@ import url_launcher_macos
 
 func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
   FilePickerPlugin.register(with: registry.registrar(forPlugin: "FilePickerPlugin"))
+  FileSelectorPlugin.register(with: registry.registrar(forPlugin: "FileSelectorPlugin"))
   FPPPackageInfoPlusPlugin.register(with: registry.registrar(forPlugin: "FPPPackageInfoPlusPlugin"))
   PrintingPlugin.register(with: registry.registrar(forPlugin: "PrintingPlugin"))
   SharedPreferencesPlugin.register(with: registry.registrar(forPlugin: "SharedPreferencesPlugin"))