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/windows/flutter/generated_plugin_registrant.cc

@@ -6,10 +6,13 @@
 
 #include "generated_plugin_registrant.h"
 
+#include <file_selector_windows/file_selector_windows.h>
 #include <printing/printing_plugin.h>
 #include <url_launcher_windows/url_launcher_windows.h>
 
 void RegisterPlugins(flutter::PluginRegistry* registry) {
+  FileSelectorWindowsRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("FileSelectorWindows"));
   PrintingPluginRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("PrintingPlugin"));
   UrlLauncherWindowsRegisterWithRegistrar(