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/pubspec.lock

@@ -423,6 +423,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  gal:
+    dependency: "direct main"
+    description:
+      name: gal
+      sha256: f71e79840fe023a21f2f949771375444b6efcd34b9e625d5f4f5504971380a77
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "2.3.3"
   glob:
     dependency: transitive
     description:
@@ -836,6 +844,22 @@ packages:
       url: "https://pub.flutter-io.cn"
     source: hosted
     version: "3.4.3"
+  share_plus:
+    dependency: "direct main"
+    description:
+      name: share_plus
+      sha256: "34f00f9becd2743c1fb05363d624f9f70d37f7ccdcdda47450bc0b8c9d327b8c"
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "13.3.0"
+  share_plus_platform_interface:
+    dependency: transitive
+    description:
+      name: share_plus_platform_interface
+      sha256: "365ef7379fc22507256adda3385152942ffce08935452bc972c2e52a0bebae41"
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "7.2.0"
   shared_preferences:
     dependency: "direct main"
     description: