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/pages/album_form_page.dart

@@ -89,16 +89,34 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
   /// 创建模式：相册创建成功后上传；编辑模式：保存后上传。
   String? _coverFilePath;
 
+  /// 用户选择的宣传图本机文件路径（null = 未选；上传时机同封面）。
+  String? _bannerFilePath;
+
+  /// 编辑模式下的现有宣传图 URL（asset 现状查询，异步加载即时预览）。
+  String? _existingBannerUrl;
+
   /// 编辑模式下的现有封面 URL（来自 AlbumSummary.cover，即时预览）。
-  String? get _existingCoverUrl => _coverFilePath == null
-      ? (widget.existing?.cover?.url)
-      : null;
+  String? get _existingCoverUrl =>
+      _coverFilePath == null ? (widget.existing?.cover?.url) : null;
 
   bool get _isCreate => widget.existing == null;
 
   @override
   void initState() {
     super.initState();
+    // 编辑模式：加载现有宣传图预览（asset 现状查询；失败静默不阻塞表单）。
+    if (widget.existing != null) {
+      Future<void>.microtask(() async {
+        try {
+          final url = await ref
+              .read(apiClientProvider)
+              .fetchAlbumAssetUrl(widget.existing!.id, 'banner');
+          if (mounted) setState(() => _existingBannerUrl = url);
+        } on Object {
+          // 预览加载失败不阻塞表单
+        }
+      });
+    }
     final initial = widget.existing == null
         ? (widget.initialDraft ??
               AlbumFormDraft(
@@ -245,7 +263,9 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
         albumId = album.id;
       } else {
         final patch = draft.toUpdatePatch(widget.existing!);
-        if (patch == null && _coverFilePath == null) {
+        if (patch == null &&
+            _coverFilePath == null &&
+            _bannerFilePath == null) {
           navigator.pop(false);
           return;
         }
@@ -269,6 +289,22 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
           );
         }
       }
+      // 宣传图上传：同封面——保存成功后执行，失败不阻塞表单结果。
+      if (_bannerFilePath != null) {
+        try {
+          await ref
+              .read(apiClientProvider)
+              .uploadAlbumAssetFromFile(
+                albumId,
+                kind: 'banner',
+                filePath: _bannerFilePath!,
+              );
+        } on Object {
+          messenger.showSnackBar(
+            SnackBar(content: Text(l10n.bannerUploadFailed)),
+          );
+        }
+      }
       navigator.pop(true);
     } on AppException catch (error) {
       // 保存失败：保留输入，仅提示。
@@ -370,7 +406,12 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
         ),
         // 保存按钮固定底部：键盘打开时常驻可达（§4.5）。
         bottomNavigationBar: SafeArea(
-          minimum: const EdgeInsets.fromLTRB(kSpace16, kSpace8, kSpace16, kSpace12),
+          minimum: const EdgeInsets.fromLTRB(
+            kSpace16,
+            kSpace8,
+            kSpace16,
+            kSpace12,
+          ),
           child: FilledButton(
             onPressed: _canSubmit ? _submit : null,
             child: _submitting
@@ -385,8 +426,8 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
         body: mainTabs == null
             ? _detailsForm(l10n)
             : _mainIndex == 0
-                ? _detailsForm(l10n)
-                : _accessPanel(l10n),
+            ? _detailsForm(l10n)
+            : _accessPanel(l10n),
       ),
     );
   }
@@ -396,8 +437,7 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
     return Column(
       crossAxisAlignment: CrossAxisAlignment.start,
       children: [
-        Text(l10n.coverPicker,
-            style: Theme.of(context).textTheme.titleMedium),
+        Text(l10n.coverPicker, style: Theme.of(context).textTheme.titleMedium),
         const SizedBox(height: kSpace8),
         InkWell(
           onTap: _submitting ? null : _pickCoverImage,
@@ -422,42 +462,43 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
                       errorBuilder: (_, _, _) => _coverPlaceholder(l10n),
                     )
                   : _existingCoverUrl != null
-                      ? Image.network(
-                          _existingCoverUrl!,
-                          fit: BoxFit.cover,
-                          errorBuilder: (_, _, _) => _coverPlaceholder(l10n),
-                        )
-                      : _coverPlaceholder(l10n),
+                  ? Image.network(
+                      _existingCoverUrl!,
+                      fit: BoxFit.cover,
+                      errorBuilder: (_, _, _) => _coverPlaceholder(l10n),
+                    )
+                  : _coverPlaceholder(l10n),
             ),
           ),
         ),
         const SizedBox(height: kSpace4),
         Text(
-          _coverFilePath != null
-              ? l10n.coverSelected
-              : l10n.coverPickerHint,
+          _coverFilePath != null ? l10n.coverSelected : l10n.coverPickerHint,
           style: Theme.of(context).textTheme.bodySmall?.copyWith(
-                color: Theme.of(context).colorScheme.onSurfaceVariant,
-              ),
+            color: Theme.of(context).colorScheme.onSurfaceVariant,
+          ),
         ),
       ],
     );
   }
 
   Widget _coverPlaceholder(AppLocalizations l10n) => Column(
-        mainAxisAlignment: MainAxisAlignment.center,
-        children: [
-          Icon(Icons.add_photo_alternate_outlined,
-              size: 48, color: Theme.of(context).colorScheme.primary),
-          const SizedBox(height: kSpace8),
-          Text(
-            l10n.coverPickerHint,
-            style: Theme.of(context).textTheme.bodySmall?.copyWith(
-                  color: Theme.of(context).colorScheme.onSurfaceVariant,
-                ),
-          ),
-        ],
-      );
+    mainAxisAlignment: MainAxisAlignment.center,
+    children: [
+      Icon(
+        Icons.add_photo_alternate_outlined,
+        size: 48,
+        color: Theme.of(context).colorScheme.primary,
+      ),
+      const SizedBox(height: kSpace8),
+      Text(
+        l10n.coverPickerHint,
+        style: Theme.of(context).textTheme.bodySmall?.copyWith(
+          color: Theme.of(context).colorScheme.onSurfaceVariant,
+        ),
+      ),
+    ],
+  );
 
   Future<void> _pickCoverImage() async {
     final path = await ref.read(singleImagePickerProvider)();
@@ -466,6 +507,81 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
     }
   }
 
+  Future<void> _pickBannerImage() async {
+    final path = await ref.read(singleImagePickerProvider)();
+    if (path != null) {
+      setState(() => _bannerFilePath = path);
+    }
+  }
+
+  /// 宣传图（banner）选择器：3:1 横幅可点击区域；编辑模式显示现有 banner。
+  Widget _buildBannerPicker(AppLocalizations l10n) {
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        Text(l10n.bannerPicker, style: Theme.of(context).textTheme.titleMedium),
+        const SizedBox(height: kSpace8),
+        InkWell(
+          onTap: _submitting ? null : _pickBannerImage,
+          borderRadius: BorderRadius.circular(kRadiusInput),
+          child: Container(
+            height: 120,
+            width: double.infinity,
+            decoration: BoxDecoration(
+              color: Theme.of(context).colorScheme.surfaceContainerHighest,
+              borderRadius: BorderRadius.circular(kRadiusInput),
+              border: Border.all(
+                color: Theme.of(context).colorScheme.outlineVariant,
+                width: 1,
+              ),
+            ),
+            child: ClipRRect(
+              borderRadius: BorderRadius.circular(kRadiusInput),
+              child: _bannerFilePath != null
+                  ? Image.file(
+                      File(_bannerFilePath!),
+                      fit: BoxFit.cover,
+                      errorBuilder: (_, _, _) => _bannerPlaceholder(l10n),
+                    )
+                  : _existingBannerUrl != null
+                  ? Image.network(
+                      _existingBannerUrl!,
+                      fit: BoxFit.cover,
+                      errorBuilder: (_, _, _) => _bannerPlaceholder(l10n),
+                    )
+                  : _bannerPlaceholder(l10n),
+            ),
+          ),
+        ),
+        const SizedBox(height: kSpace4),
+        Text(
+          _bannerFilePath != null ? l10n.bannerSelected : l10n.bannerPickerHint,
+          style: Theme.of(context).textTheme.bodySmall?.copyWith(
+            color: Theme.of(context).colorScheme.onSurfaceVariant,
+          ),
+        ),
+      ],
+    );
+  }
+
+  Widget _bannerPlaceholder(AppLocalizations l10n) => Column(
+    mainAxisAlignment: MainAxisAlignment.center,
+    children: [
+      Icon(
+        Icons.panorama_outlined,
+        size: 40,
+        color: Theme.of(context).colorScheme.primary,
+      ),
+      const SizedBox(height: kSpace8),
+      Text(
+        l10n.bannerPickerHint,
+        style: Theme.of(context).textTheme.bodySmall?.copyWith(
+          color: Theme.of(context).colorScheme.onSurfaceVariant,
+        ),
+      ),
+    ],
+  );
+
   /// Details（= 创建表单）：封面选择 + 分组字段 + 校验错误汇总。
   Widget _detailsForm(AppLocalizations l10n) {
     return Form(
@@ -477,6 +593,10 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
             children: [
               // ── 封面选择（16:9 可点击区域） ─────────────────
               _buildCoverPicker(l10n),
+              const SizedBox(height: kSpace16),
+
+              // ── 宣传图选择（3:1 横幅；编辑模式显示现有 banner）──
+              _buildBannerPicker(l10n),
               const SizedBox(height: kSpace24),
 
               // ── 分组 1：Event details ──────────────────────────