added a swipeable media gallery for chat and folder images and videos with server previews, on-demand originals and zoom, and marked document previews in chat so they no longer pass for photos

lib/widget/media_gallery/gallery_source.dart

@@ -0,0 +1,21 @@
+import 'package:flutter/foundation.dart';
+
+import 'gallery_item.dart';
+
+/// Items the media gallery pages through. Sources that grow while the viewer
+/// is open (chat history) notify; the viewer keeps the shown item in place.
+abstract class GallerySource extends ChangeNotifier {
+  List<GalleryItem> get items;
+
+  bool get hasOlder => false;
+
+  /// Called when the user nears the start (oldest end) of [items].
+  Future<void> loadOlder() async {}
+}
+
+class StaticGallerySource extends GallerySource {
+  @override
+  final List<GalleryItem> items;
+
+  StaticGallerySource(this.items);
+}