fix(download): make picker drag dismissal natural

lib/widgets/app_bottom_sheet.dart

@@ -40,6 +40,39 @@ class AppSheetHandle extends StatelessWidget {
   }
 }
 
+/// A modal-sheet surface whose height and inner scroll position share one drag.
+///
+/// The child must attach the supplied [ScrollController] to its primary
+/// vertical scroll view. Pulling down at the top then moves the whole surface;
+/// releasing either restores it or dismisses it at the minimum extent.
+class AppDraggableSheet extends StatelessWidget {
+  const AppDraggableSheet({
+    super.key,
+    required this.builder,
+    this.initialChildSize = 0.88,
+    this.minChildSize = 0.25,
+    this.maxChildSize = 0.88,
+  });
+
+  final ScrollableWidgetBuilder builder;
+  final double initialChildSize;
+  final double minChildSize;
+  final double maxChildSize;
+
+  @override
+  Widget build(BuildContext context) {
+    return DraggableScrollableSheet(
+      expand: false,
+      snap: true,
+      shouldCloseOnMinExtent: true,
+      initialChildSize: initialChildSize,
+      minChildSize: minChildSize,
+      maxChildSize: maxChildSize,
+      builder: builder,
+    );
+  }
+}
+
 /// Standard chrome for modal sheet content: drag handle, optional title block,
 /// height cap, keyboard inset and bottom safe area, so each sheet only supplies
 /// its own body.