feat(app): crop cover & banner before upload (crop_your_image, 16:9 / 3:1)

review fixes: drop GIF from pre-sniff (package maps gif→png decode → never-ready hang); PopScope blocks back during processing

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

apps/mobile/lib/pages/album_form_page.dart

@@ -31,6 +31,7 @@ import '../features/uploads/enqueue.dart';
 import '../l10n/app_localizations.dart';
 import '../ui/components/section_header.dart';
 import '../ui/tokens.dart';
+import 'image_crop_page.dart';
 
 /// 创建（[existing] 为 null）/ 编辑相册表单。
 class AlbumFormPage extends ConsumerStatefulWidget {
@@ -497,17 +498,43 @@ class _AlbumFormPageState extends ConsumerState<AlbumFormPage>
     ],
   );
 
+  /// 选图后必经裁剪：封面 16:9（表单预览区比例），输出宽 ≤1920。
   Future<void> _pickCoverImage() async {
     final path = await ref.read(singleImagePickerProvider)();
-    if (path != null) {
-      setState(() => _coverFilePath = path);
+    if (path == null || !mounted) {
+      return;
+    }
+    final cropped = await Navigator.of(context).push<String>(
+      MaterialPageRoute(
+        builder: (_) => ImageCropPage(
+          sourcePath: path,
+          aspectRatio: 16 / 9,
+          maxOutputWidth: 1920,
+        ),
+      ),
+    );
+    if (cropped != null && mounted) {
+      setState(() => _coverFilePath = cropped);
     }
   }
 
+  /// 选图后必经裁剪：宣传图 3:1（观众页横幅），输出宽 ≤2400。
   Future<void> _pickBannerImage() async {
     final path = await ref.read(singleImagePickerProvider)();
-    if (path != null) {
-      setState(() => _bannerFilePath = path);
+    if (path == null || !mounted) {
+      return;
+    }
+    final cropped = await Navigator.of(context).push<String>(
+      MaterialPageRoute(
+        builder: (_) => ImageCropPage(
+          sourcePath: path,
+          aspectRatio: 3 / 1,
+          maxOutputWidth: 2400,
+        ),
+      ),
+    );
+    if (cropped != null && mounted) {
+      setState(() => _bannerFilePath = cropped);
     }
   }