feat(app): album banner picker in edit form — show current banner, pick & upload on save

- api_client：uploadAlbumAssetFromFile 泛化（cover 为薄壳）+ fetchAlbumAssetUrl
  （GET /albums/{id}/assets → 指定 kind 全局行 URL）
- 编辑页 Details：封面下方新增「宣传图」3:1 选择器——编辑模式异步加载现有
  banner 预览（失败静默）；保存成功后上传（同封面：失败不阻塞表单结果）
- 未保存变更判断纳入 banner；l10n zh/en
- 测试适配：编辑用例 pumpForm 后清空 banner 现状预检请求（397/397 全绿）

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

apps/mobile/lib/l10n/app_localizations.dart

@@ -1957,6 +1957,30 @@ abstract class AppLocalizations {
   /// In zh, this message translates to:
   /// **'相册已删除'**
   String get albumDeleted;
+
+  /// No description provided for @bannerPicker.
+  ///
+  /// In zh, this message translates to:
+  /// **'宣传图'**
+  String get bannerPicker;
+
+  /// No description provided for @bannerPickerHint.
+  ///
+  /// In zh, this message translates to:
+  /// **'点击选择宣传图（展示在观众页顶部）'**
+  String get bannerPickerHint;
+
+  /// No description provided for @bannerSelected.
+  ///
+  /// In zh, this message translates to:
+  /// **'已选择宣传图——保存时上传'**
+  String get bannerSelected;
+
+  /// No description provided for @bannerUploadFailed.
+  ///
+  /// In zh, this message translates to:
+  /// **'宣传图上传失败'**
+  String get bannerUploadFailed;
 }
 
 class _AppLocalizationsDelegate