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/pubspec.lock

@@ -161,6 +161,38 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "8.3.7"
+  file_selector_linux:
+    dependency: transitive
+    description:
+      name: file_selector_linux
+      sha256: "2567f398e06ac72dcf2e98a0c95df2a9edd03c2c2e0cacd4780f20cdf56263a0"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.9.4"
+  file_selector_macos:
+    dependency: transitive
+    description:
+      name: file_selector_macos
+      sha256: "5e0bbe9c312416f1787a68259ea1505b52f258c587f12920422671807c4d618a"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.9.5"
+  file_selector_platform_interface:
+    dependency: transitive
+    description:
+      name: file_selector_platform_interface
+      sha256: "35e0bd61ebcdb91a3505813b055b09b79dfdc7d0aee9c09a7ba59ae4bb13dc85"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.7.0"
+  file_selector_windows:
+    dependency: transitive
+    description:
+      name: file_selector_windows
+      sha256: "62197474ae75893a62df75939c777763d39c2bc5f73ce5b88497208bc269abfd"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.9.3+5"
   flutter:
     dependency: "direct main"
     description: flutter
@@ -253,6 +285,70 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "4.3.0"
+  image_picker:
+    dependency: "direct main"
+    description:
+      name: image_picker
+      sha256: "91c025426c2881c551100bce834e201c835a170151545f58d17da5180ca7d9ac"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.2.2"
+  image_picker_android:
+    dependency: transitive
+    description:
+      name: image_picker_android
+      sha256: "6f3a1995eafb000333174fae92202622033b0ee7fd917a6cd3730295264df84a"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.8.13+19"
+  image_picker_for_web:
+    dependency: transitive
+    description:
+      name: image_picker_for_web
+      sha256: "66257a3191ab360d23a55c8241c91a6e329d31e94efa7be9cf7a212e65850214"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.1.1"
+  image_picker_ios:
+    dependency: transitive
+    description:
+      name: image_picker_ios
+      sha256: b9c4a438a9ff4f60808c9cf0039b93a42bb6c2211ef6ebb647394b2b3fa84588
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.8.13+6"
+  image_picker_linux:
+    dependency: transitive
+    description:
+      name: image_picker_linux
+      sha256: "1f81c5f2046b9ab724f85523e4af65be1d47b038160a8c8deed909762c308ed4"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.2.2"
+  image_picker_macos:
+    dependency: transitive
+    description:
+      name: image_picker_macos
+      sha256: "86f0f15a309de7e1a552c12df9ce5b59fe927e71385329355aec4776c6a8ec91"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.2.2+1"
+  image_picker_platform_interface:
+    dependency: transitive
+    description:
+      name: image_picker_platform_interface
+      sha256: "567e056716333a1647c64bb6bd873cff7622233a5c3f694be28a583d4715690c"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.11.1"
+  image_picker_windows:
+    dependency: transitive
+    description:
+      name: image_picker_windows
+      sha256: d248c86554a72b5495a31c56f060cf73a41c7ff541689327b1a7dbccc33adfae
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.2.2"
   intl:
     dependency: "direct main"
     description: