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/lib/screens/products/product_detail_screen.dart

@@ -1,6 +1,7 @@
 import '../../core/utils/dialog_util.dart';
 import 'dart:typed_data';
 import 'package:file_picker/file_picker.dart';
+import 'package:image_picker/image_picker.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
@@ -91,7 +92,56 @@ class _ProductDetailScreenState extends ConsumerState<ProductDetailScreen> {
     }
   }
 
+  // 移动端（Android/iOS）支持拍照，用 image_picker；桌面/Web 用 file_picker 选文件。
+  // 用 defaultTargetPlatform 而非 dart:io 的 Platform，避免在参与 Web 编译的文件里引入 dart:io。
+  bool get _isMobile =>
+      !kIsWeb &&
+      (defaultTargetPlatform == TargetPlatform.android ||
+          defaultTargetPlatform == TargetPlatform.iOS);
+
   Future<void> _pickAndUpload() async {
+    if (_isMobile) {
+      await _pickFromCameraOrGallery();
+    } else {
+      await _pickFromFiles();
+    }
+  }
+
+  /// 移动端：底部弹出「拍照 / 从相册选择」
+  Future<void> _pickFromCameraOrGallery() async {
+    final source = await showModalBottomSheet<ImageSource>(
+      context: context,
+      builder: (ctx) => SafeArea(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            ListTile(
+              leading: const Icon(Icons.camera_alt_outlined),
+              title: const Text('拍照'),
+              onTap: () => Navigator.pop(ctx, ImageSource.camera),
+            ),
+            ListTile(
+              leading: const Icon(Icons.photo_library_outlined),
+              title: const Text('从相册选择'),
+              onTap: () => Navigator.pop(ctx, ImageSource.gallery),
+            ),
+          ],
+        ),
+      ),
+    );
+    if (source == null) return;
+
+    final XFile? xfile = await ImagePicker().pickImage(
+      source: source,
+      maxWidth: 2000,
+      imageQuality: 85,
+    );
+    if (xfile == null) return;
+    await _uploadImage(filePath: xfile.path, fileName: xfile.name);
+  }
+
+  /// 桌面 / Web：文件选择器
+  Future<void> _pickFromFiles() async {
     final result = await FilePicker.platform.pickFiles(
       type: FileType.image,
       allowMultiple: false,
@@ -102,13 +152,17 @@ class _ProductDetailScreenState extends ConsumerState<ProductDetailScreen> {
     final String? filePath = kIsWeb ? null : file.path;
     final fileBytes = file.bytes;
     if (filePath == null && fileBytes == null) return;
+    await _uploadImage(
+        filePath: filePath, bytes: fileBytes, fileName: file.name);
+  }
 
+  Future<void> _uploadImage(
+      {String? filePath, Uint8List? bytes, required String fileName}) async {
     setState(() => _uploading = true);
     try {
-      final img = await ref
-          .read(productRepositoryProvider)
-          .uploadImage(_product!.id, filePath,
-              bytes: fileBytes, fileName: file.name);
+      final img = await ref.read(productRepositoryProvider).uploadImage(
+          _product!.id, filePath,
+          bytes: bytes, fileName: fileName);
       _updateImages([..._product!.images, img]);
       if (mounted) {
         ScaffoldMessenger.of(context).showSnackBar(