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",