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"
 }