feat(mobile-app): change share QR code to APK download link with referral code

QR code now contains: https://s3.szaiai.com/rwadurian/app-release.apk?ref={referralCode}
- User scans QR to download APK directly
- Guide page 5 already supports parsing ref parameter from URL

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

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

frontend/mobile-app/lib/features/share/presentation/pages/share_page.dart

@@ -38,7 +38,10 @@ class _SharePageState extends ConsumerState<SharePage> {
     _loadShareLink();
   }
 
-  /// 加载分享链接 (调用 API 生成短链)
+  /// APK 下载基础链接
+  static const String _apkDownloadUrl = 'https://s3.szaiai.com/rwadurian/app-release.apk';
+
+  /// 加载分享链接 (使用 APK 下载链接 + 推荐码)
   Future<void> _loadShareLink() async {
     try {
       setState(() {
@@ -48,15 +51,14 @@ class _SharePageState extends ConsumerState<SharePage> {
 
       final referralService = ref.read(referralServiceProvider);
 
-      // 调用 API 生成短链
+      // 调用 API 获取推荐码
       final linkResponse = await referralService.generateReferralLink();
 
       if (mounted) {
         setState(() {
-          // 优先使用短链，如果没有则使用完整链接
-          _displayLink = linkResponse.shortUrl.isNotEmpty
-              ? linkResponse.shortUrl
-              : linkResponse.fullUrl;
+          // 使用 APK 下载链接 + 推荐码参数
+          final referralCode = linkResponse.referralCode;
+          _displayLink = '$_apkDownloadUrl?ref=$referralCode';
           _isLoading = false;
         });
       }
@@ -65,9 +67,13 @@ class _SharePageState extends ConsumerState<SharePage> {
       if (mounted) {
         setState(() {
           _isLoading = false;
-          // 失败时使用传入的默认链接
-          _displayLink = widget.shareLink;
-          _errorMessage = '加载短链失败，使用默认链接';
+          // 失败时使用传入的推荐码构造链接
+          if (widget.referralCode != null && widget.referralCode!.isNotEmpty) {
+            _displayLink = '$_apkDownloadUrl?ref=${widget.referralCode}';
+          } else {
+            _displayLink = _apkDownloadUrl;
+          }
+          _errorMessage = '加载失败，使用默认链接';
         });
       }
     }