feat(mobile-app): add gallery image selection for QR code scanning

- Add image_picker import for gallery access
- Implement _pickImageAndScan() method to select and analyze images
- Add "从相册选择" button in QR scanner UI with loading state
- Show SnackBar feedback when QR code not found in image

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/mobile-app/lib/features/auth/presentation/pages/guide_page.dart

@@ -3,6 +3,7 @@ import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_screenutil/flutter_screenutil.dart';
 import 'package:go_router/go_router.dart';
+import 'package:image_picker/image_picker.dart';
 import 'package:mobile_scanner/mobile_scanner.dart';
 import '../../../../routes/route_paths.dart';
 import '../providers/auth_provider.dart';
@@ -596,6 +597,7 @@ class _QrScannerPageState extends State<_QrScannerPage> {
   MobileScannerController? _controller;
   bool _hasScanned = false;
   bool _torchOn = false;
+  bool _isProcessingImage = false;
 
   @override
   void initState() {
@@ -632,6 +634,84 @@ class _QrScannerPageState extends State<_QrScannerPage> {
     });
   }
 
+  /// 从相册选择图片并扫描二维码
+  Future<void> _pickImageAndScan() async {
+    if (_isProcessingImage || _hasScanned) return;
+
+    setState(() {
+      _isProcessingImage = true;
+    });
+
+    try {
+      final ImagePicker picker = ImagePicker();
+      final XFile? image = await picker.pickImage(source: ImageSource.gallery);
+
+      if (image == null) {
+        setState(() {
+          _isProcessingImage = false;
+        });
+        return;
+      }
+
+      // 使用 MobileScannerController 分析图片
+      final BarcodeCapture? result = await _controller?.analyzeImage(image.path);
+
+      if (result != null && result.barcodes.isNotEmpty) {
+        for (final barcode in result.barcodes) {
+          if (barcode.rawValue != null && barcode.rawValue!.isNotEmpty) {
+            _hasScanned = true;
+            if (mounted) {
+              Navigator.of(context).pop(barcode.rawValue);
+            }
+            return;
+          }
+        }
+      }
+
+      // 未识别到二维码
+      if (mounted) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          SnackBar(
+            content: Text(
+              '未能识别图片中的二维码，请重新选择',
+              style: TextStyle(fontSize: 14.sp),
+            ),
+            backgroundColor: const Color(0xFF6F6354),
+            behavior: SnackBarBehavior.floating,
+            margin: EdgeInsets.all(16.w),
+            shape: RoundedRectangleBorder(
+              borderRadius: BorderRadius.circular(8.r),
+            ),
+          ),
+        );
+      }
+    } catch (e) {
+      debugPrint('扫描图片失败: $e');
+      if (mounted) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          SnackBar(
+            content: Text(
+              '图片扫描失败，请重试',
+              style: TextStyle(fontSize: 14.sp),
+            ),
+            backgroundColor: Colors.red,
+            behavior: SnackBarBehavior.floating,
+            margin: EdgeInsets.all(16.w),
+            shape: RoundedRectangleBorder(
+              borderRadius: BorderRadius.circular(8.r),
+            ),
+          ),
+        );
+      }
+    } finally {
+      if (mounted) {
+        setState(() {
+          _isProcessingImage = false;
+        });
+      }
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
@@ -669,18 +749,66 @@ class _QrScannerPageState extends State<_QrScannerPage> {
           ),
           // 扫描框遮罩
           _buildScanOverlay(),
-          // 底部提示
+          // 底部区域：提示文字和相册按钮
           Positioned(
-            bottom: 100.h,
+            bottom: 60.h,
             left: 0,
             right: 0,
-            child: Text(
+            child: Column(
-              '将二维码放入框内，即可自动扫描',
+              children: [
-              style: TextStyle(
+                Text(
-                fontSize: 14.sp,
+                  '将二维码放入框内，即可自动扫描',
-                color: Colors.white70,
+                  style: TextStyle(
-              ),
+                    fontSize: 14.sp,
-              textAlign: TextAlign.center,
+                    color: Colors.white70,
+                  ),
+                  textAlign: TextAlign.center,
+                ),
+                SizedBox(height: 32.h),
+                // 相册按钮
+                GestureDetector(
+                  onTap: _isProcessingImage ? null : _pickImageAndScan,
+                  child: Container(
+                    padding: EdgeInsets.symmetric(horizontal: 24.w, vertical: 12.h),
+                    decoration: BoxDecoration(
+                      color: Colors.white.withValues(alpha: 0.15),
+                      borderRadius: BorderRadius.circular(24.r),
+                      border: Border.all(
+                        color: Colors.white.withValues(alpha: 0.3),
+                        width: 1,
+                      ),
+                    ),
+                    child: Row(
+                      mainAxisSize: MainAxisSize.min,
+                      children: [
+                        if (_isProcessingImage)
+                          SizedBox(
+                            width: 18.sp,
+                            height: 18.sp,
+                            child: const CircularProgressIndicator(
+                              strokeWidth: 2,
+                              valueColor: AlwaysStoppedAnimation<Color>(Colors.white),
+                            ),
+                          )
+                        else
+                          Icon(
+                            Icons.photo_library_outlined,
+                            size: 18.sp,
+                            color: Colors.white,
+                          ),
+                        SizedBox(width: 8.w),
+                        Text(
+                          _isProcessingImage ? '识别中...' : '从相册选择',
+                          style: TextStyle(
+                            fontSize: 14.sp,
+                            color: Colors.white,
+                          ),
+                        ),
+                      ],
+                    ),
+                  ),
+                ),
+              ],
             ),
           ),
         ],