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

test/widget/media_gallery_test.dart

@@ -0,0 +1,82 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:marianum_mobile/share_intent/remote_file_ref.dart';
+import 'package:marianum_mobile/widget/media_gallery/gallery_item.dart';
+
+GalleryItem _chat(int id, {String name = 'a.jpg'}) => GalleryItem(
+  file: RemoteFileRef(path: 'Talk/$name', name: name),
+  kind: GalleryMediaKind.image,
+  messageId: id,
+);
+
+GalleryItem _file(String name) => GalleryItem(
+  file: RemoteFileRef(path: '/Fotos/$name', name: name),
+  kind: GalleryMediaKind.image,
+);
+
+void main() {
+  group('galleryMediaKindOf', () {
+    test('classifies images and videos case-insensitively', () {
+      expect(galleryMediaKindOf('Foto.JPG'), GalleryMediaKind.image);
+      expect(galleryMediaKindOf('scan.heic'), GalleryMediaKind.image);
+      expect(galleryMediaKindOf('clip.mov'), GalleryMediaKind.video);
+    });
+
+    test('prefers the MIME type over the extension', () {
+      expect(
+        galleryMediaKindOf('IMG_0042', mimeType: 'image/jpeg'),
+        GalleryMediaKind.image,
+      );
+      expect(
+        galleryMediaKindOf('aufnahme.bin', mimeType: 'video/mp4'),
+        GalleryMediaKind.video,
+      );
+      expect(galleryMediaKindOf('scan.jfif'), GalleryMediaKind.image);
+    });
+
+    test('skips image types without a usable preview', () {
+      expect(
+        galleryMediaKindOf(
+          'entwurf.psd',
+          mimeType: 'image/vnd.adobe.photoshop',
+        ),
+        isNull,
+      );
+    });
+
+    test('ignores everything else', () {
+      expect(galleryMediaKindOf('Arbeitsblatt.pdf'), isNull);
+      expect(galleryMediaKindOf('logo.svg'), isNull);
+      expect(galleryMediaKindOf('scan.tiff', mimeType: 'image/tiff'), isNull);
+      expect(galleryMediaKindOf('README'), isNull);
+    });
+  });
+
+  group('mergeGalleryItems', () {
+    test('sorts chat media chronologically and drops duplicates', () {
+      final merged = mergeGalleryItems(
+        [_chat(50)],
+        [_chat(70), _chat(50), _chat(30)],
+      );
+      expect(merged.map((i) => i.messageId), [30, 50, 70]);
+    });
+
+    test('keeps the existing instance for a known key', () {
+      final initial = _chat(50);
+      final merged = mergeGalleryItems([initial], [_chat(50, name: 'b.jpg')]);
+      expect(merged.single, same(initial));
+    });
+
+    test('keeps folder order untouched', () {
+      final merged = mergeGalleryItems(
+        [_file('c.jpg'), _file('a.jpg')],
+        [_file('b.jpg')],
+      );
+      expect(merged.map((i) => i.file.name), ['c.jpg', 'a.jpg', 'b.jpg']);
+    });
+  });
+
+  test('GIFs always need the original', () {
+    expect(_chat(1, name: 'lol.GIF').needsOriginal, isTrue);
+    expect(_chat(1).needsOriginal, isFalse);
+  });
+}