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/pubspec.lock

@@ -418,6 +418,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  fluwx:
+    dependency: "direct main"
+    description:
+      name: fluwx
+      sha256: "4b526d281be8560a490bd1b945373a23ab2a12088c8fc5997ffb31cc6fb41082"
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "6.0.2"
   fuchsia_remote_debug_protocol:
     dependency: transitive
     description: flutter