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_en.arb

@@ -417,5 +417,9 @@
   "deleteAlbum": "Delete album",
   "deleteAlbumConfirmTitle": "Delete this album?",
   "deleteAlbumConfirmBody": "The album becomes invisible immediately and cannot be restored. Data is retained; recovery requires operations.",
-  "albumDeleted": "Album deleted"
+  "albumDeleted": "Album deleted",
+  "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"
 }

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

apps/mobile/lib/l10n/app_localizations_en.dart

@@ -1000,4 +1000,17 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get albumDeleted => 'Album deleted';
+
+  @override
+  String get bannerPicker => 'Banner';
+
+  @override
+  String get bannerPickerHint =>
+      'Tap to pick a banner image (shown atop the viewer page)';
+
+  @override
+  String get bannerSelected => 'Banner selected — will upload on save';
+
+  @override
+  String get bannerUploadFailed => 'Banner upload failed';
 }

apps/mobile/lib/l10n/app_localizations_zh.dart

@@ -968,4 +968,16 @@ class AppLocalizationsZh extends AppLocalizations {
 
   @override
   String get albumDeleted => '相册已删除';
+
+  @override
+  String get bannerPicker => '宣传图';
+
+  @override
+  String get bannerPickerHint => '点击选择宣传图（展示在观众页顶部）';
+
+  @override
+  String get bannerSelected => '已选择宣传图——保存时上传';
+
+  @override
+  String get bannerUploadFailed => '宣传图上传失败';
 }

apps/mobile/lib/l10n/app_zh.arb

@@ -417,5 +417,9 @@
   "deleteAlbum": "删除相册",
   "deleteAlbumConfirmTitle": "删除这个相册？",
   "deleteAlbumConfirmBody": "删除后立即不可见且不可恢复。数据保留，恢复需运维处理。",
-  "albumDeleted": "相册已删除"
+  "albumDeleted": "相册已删除",
+  "bannerPicker": "宣传图",
+  "bannerPickerHint": "点击选择宣传图（展示在观众页顶部）",
+  "bannerSelected": "已选择宣传图——保存时上传",
+  "bannerUploadFailed": "宣传图上传失败"
 }