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_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';
 }