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_en.arb

@@ -421,5 +421,10 @@
   "bannerPicker": "Banner",
   "bannerPickerHint": "Tap to pick a banner image (shown atop the viewer page)",
   "bannerSelected": "Banner selected — will upload on save",
-  "bannerUploadFailed": "Banner upload failed"
+  "bannerUploadFailed": "Banner upload failed",
+  "shareCardMode": "Share to WeChat / other apps",
+  "sharePosterMode": "Generate QR poster",
+  "posterDownload": "Download poster image",
+  "posterSaved": "Poster saved to gallery",
+  "posterSaveFailed": "Failed to save poster"
 }

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

apps/mobile/lib/l10n/app_localizations_en.dart

@@ -1013,4 +1013,19 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get bannerUploadFailed => 'Banner upload failed';
+
+  @override
+  String get shareCardMode => 'Share to WeChat / other apps';
+
+  @override
+  String get sharePosterMode => 'Generate QR poster';
+
+  @override
+  String get posterDownload => 'Download poster image';
+
+  @override
+  String get posterSaved => 'Poster saved to gallery';
+
+  @override
+  String get posterSaveFailed => 'Failed to save poster';
 }

apps/mobile/lib/l10n/app_localizations_zh.dart

@@ -980,4 +980,19 @@ class AppLocalizationsZh extends AppLocalizations {
 
   @override
   String get bannerUploadFailed => '宣传图上传失败';
+
+  @override
+  String get shareCardMode => '分享到微信 / 其他应用';
+
+  @override
+  String get sharePosterMode => '生成二维码海报';
+
+  @override
+  String get posterDownload => '下载海报图';
+
+  @override
+  String get posterSaved => '海报已保存到相册';
+
+  @override
+  String get posterSaveFailed => '海报保存失败';
 }

apps/mobile/lib/l10n/app_zh.arb

@@ -421,5 +421,10 @@
   "bannerPicker": "宣传图",
   "bannerPickerHint": "点击选择宣传图（展示在观众页顶部）",
   "bannerSelected": "已选择宣传图——保存时上传",
-  "bannerUploadFailed": "宣传图上传失败"
+  "bannerUploadFailed": "宣传图上传失败",
+  "shareCardMode": "分享到微信 / 其他应用",
+  "sharePosterMode": "生成二维码海报",
+  "posterDownload": "下载海报图",
+  "posterSaved": "海报已保存到相册",
+  "posterSaveFailed": "海报保存失败"
 }