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_item.dart

@@ -0,0 +1,79 @@
+import '../../share_intent/remote_file_ref.dart';
+import '../file_viewer/file_kind.dart';
+
+enum GalleryMediaKind { image, video }
+
+/// Single rule for "is this a picture/video": the gallery and the chat bubble
+/// both use it, so whatever looks like media in a chat is also in the gallery.
+GalleryMediaKind? galleryMediaKindOf(String name, {String? mimeType}) {
+  final mime = mimeType?.toLowerCase() ?? '';
+  // Only formats the device decodes itself — image/* also covers PSD, SVG,
+  // TIFF …, which would hinge on a server preview.
+  if (_galleryImageMimes.contains(mime)) return GalleryMediaKind.image;
+  if (mime.startsWith('video/')) return GalleryMediaKind.video;
+  if (isGalleryImageName(name)) return GalleryMediaKind.image;
+  if (isGalleryVideoName(name)) return GalleryMediaKind.video;
+  return null;
+}
+
+const Set<String> _galleryImageMimes = {
+  'image/jpeg',
+  'image/png',
+  'image/gif',
+  'image/webp',
+  'image/bmp',
+  'image/x-ms-bmp',
+  'image/heic',
+  'image/heif',
+  'image/avif',
+  'image/x-icon',
+  'image/vnd.microsoft.icon',
+};
+
+class GalleryItem {
+  final RemoteFileRef file;
+  final GalleryMediaKind kind;
+
+  /// Talk message id: orders chat media and dedupes overlapping pages.
+  final int? messageId;
+
+  /// Second line in the app bar, e.g. sender and date.
+  final String? subtitle;
+
+  /// Part of the cache key so an overwritten file is not served stale.
+  final String? eTag;
+
+  const GalleryItem({
+    required this.file,
+    required this.kind,
+    this.messageId,
+    this.subtitle,
+    this.eTag,
+  });
+
+  String get key => messageId?.toString() ?? file.path;
+
+  bool get isVideo => kind == GalleryMediaKind.video;
+
+  String get _extension => file.name.split('.').last.toLowerCase();
+
+  /// Server previews of a GIF are a single still frame.
+  bool get needsOriginal => _extension == 'gif';
+}
+
+/// Adds [incoming] to [current], dropping duplicates by [GalleryItem.key] and
+/// keeping chat media in chronological (message id) order.
+List<GalleryItem> mergeGalleryItems(
+  List<GalleryItem> current,
+  Iterable<GalleryItem> incoming,
+) {
+  final byKey = {for (final item in current) item.key: item};
+  for (final item in incoming) {
+    byKey.putIfAbsent(item.key, () => item);
+  }
+  final merged = byKey.values.toList();
+  if (merged.every((item) => item.messageId != null)) {
+    merged.sort((a, b) => a.messageId!.compareTo(b.messageId!));
+  }
+  return merged;
+}