feat(app,viewer): WeChat card share + 'Share to...' label + site og tags

- 按钮文字「分享到微信/其他应用」→「分享到...」（用户需求）
- 微信卡片分享（fluwx 6）：WECHAT_APPID 经 --dart-define 注入；已配置且
  装微信 → WXWebpage 卡片（标题=相册名、描述=日期·地点、缩略图=封面压缩
  240px PNG ≤32KB、点击进观众链接）；未配置/未装 → 回退系统分享面板
- viewer index.html 加站点级 og 标签（微信/QQ 抓取链接预览用；每相册动态
  og 需服务端注入，登记后续增强）
- 397/397 全绿、analyze 0

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

apps/mobile/lib/l10n/app_en.arb

@@ -422,7 +422,7 @@
   "bannerPickerHint": "Tap to pick a banner image (shown atop the viewer page)",
   "bannerSelected": "Banner selected — will upload on save",
   "bannerUploadFailed": "Banner upload failed",
-  "shareCardMode": "Share to WeChat / other apps",
+  "shareCardMode": "Share to...",
   "sharePosterMode": "Generate QR poster",
   "posterDownload": "Download poster image",
   "posterSaved": "Poster saved to gallery",

apps/mobile/lib/l10n/app_localizations.dart

@@ -1985,7 +1985,7 @@ abstract class AppLocalizations {
   /// No description provided for @shareCardMode.
   ///
   /// In zh, this message translates to:
-  /// **'分享到微信 / 其他应用'**
+  /// **'分享到...'**
   String get shareCardMode;
 
   /// No description provided for @sharePosterMode.

apps/mobile/lib/l10n/app_localizations_en.dart

@@ -1015,7 +1015,7 @@ class AppLocalizationsEn extends AppLocalizations {
   String get bannerUploadFailed => 'Banner upload failed';
 
   @override
-  String get shareCardMode => 'Share to WeChat / other apps';
+  String get shareCardMode => 'Share to...';
 
   @override
   String get sharePosterMode => 'Generate QR poster';

apps/mobile/lib/l10n/app_localizations_zh.dart

@@ -982,7 +982,7 @@ class AppLocalizationsZh extends AppLocalizations {
   String get bannerUploadFailed => '宣传图上传失败';
 
   @override
-  String get shareCardMode => '分享到微信 / 其他应用';
+  String get shareCardMode => '分享到...';
 
   @override
   String get sharePosterMode => '生成二维码海报';

apps/mobile/lib/l10n/app_zh.arb

@@ -422,7 +422,7 @@
   "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",
-  "shareCardMode": "分享到微信 / 其他应用",
+  "shareCardMode": "分享到...",
   "sharePosterMode": "生成二维码海报",
   "posterDownload": "下载海报图",
   "posterSaved": "海报已保存到相册",