feat(app): share redesign — card share (system), QR poster (banner+title+time+venue+QR, downloadable), copy link

- 三方式（用户需求）：①卡片模式默认=系统分享面板（share_plus SharePlus.
  instance，微信/其他应用，文本=标题+链接）②生成二维码=海报弹层——顶部
  宣传图（banner→cover→品牌占位回退）+ 活动标题 + 时间 + 地址 + 二维码，
  「下载海报图」RepaintBoundary 3x 渲染 PNG → gal 存相册 ③复制链接
- 依赖 share_plus + gal；分享/保存均 Provider 注入（测试替身）
- 测试适配三方式结构 + 海报弹层断言（397/397 全绿、analyze 0）

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

apps/mobile/lib/l10n/app_localizations.dart

@@ -1981,6 +1981,36 @@ abstract class AppLocalizations {
   /// In zh, this message translates to:
   /// **'宣传图上传失败'**
   String get bannerUploadFailed;
+
+  /// No description provided for @shareCardMode.
+  ///
+  /// In zh, this message translates to:
+  /// **'分享到微信 / 其他应用'**
+  String get shareCardMode;
+
+  /// No description provided for @sharePosterMode.
+  ///
+  /// In zh, this message translates to:
+  /// **'生成二维码海报'**
+  String get sharePosterMode;
+
+  /// No description provided for @posterDownload.
+  ///
+  /// In zh, this message translates to:
+  /// **'下载海报图'**
+  String get posterDownload;
+
+  /// No description provided for @posterSaved.
+  ///
+  /// In zh, this message translates to:
+  /// **'海报已保存到相册'**
+  String get posterSaved;
+
+  /// No description provided for @posterSaveFailed.
+  ///
+  /// In zh, this message translates to:
+  /// **'海报保存失败'**
+  String get posterSaveFailed;
 }
 
 class _AppLocalizationsDelegate