feat: long press no grid do mural abre o viewer de imagem em tela cheia

Toque simples segue abrindo o showModalBottomSheet com o post; long press
abre o mesmo ImageViewer das imagens de sala, com pin-to-zoom e deslize
entre as midias do mural.

lib/pages/mural/mural_chat_page.dart

@@ -14,6 +14,7 @@ import 'package:tatchat/config/themes.dart';
 import 'package:tatchat/l10n/l10n.dart';
 import 'package:tatchat/pages/chat/send_file_dialog.dart';
 import 'package:tatchat/pages/chat/sticker_picker_dialog.dart';
+import 'package:tatchat/pages/image_viewer/image_viewer.dart';
 import 'package:tatchat/pages/image_viewer/video_player.dart';
 import 'package:tatchat/utils/adaptive_bottom_sheet.dart';
 import 'package:tatchat/utils/matrix_sdk_extensions/matrix_locals.dart';
@@ -306,6 +307,19 @@ class _MuralChatViewState extends State<MuralChatView> {
     );
   }
 
+  /// Long press on a grid cell: open the same fullscreen viewer a room image
+  /// uses, instead of the bottom sheet, so pinch-to-zoom and swiping between
+  /// the media of the mural are available there too.
+  void _openMediaFullscreen(Event event) {
+    final timeline = this.timeline;
+    if (timeline == null) return;
+    showDialog<void>(
+      context: context,
+      builder: (_) =>
+          ImageViewer(event, timeline: timeline, outerContext: context),
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
@@ -395,6 +409,7 @@ class _MuralChatViewState extends State<MuralChatView> {
                         client: room.client,
                         cellSize: gridCellSize,
                         onTap: () => _openMedia(event),
+                        onLongPress: () => _openMediaFullscreen(event),
                       );
                     },
                   ),
@@ -656,12 +671,14 @@ class _MediaCell extends StatelessWidget {
   final Client client;
   final double cellSize;
   final VoidCallback onTap;
+  final VoidCallback onLongPress;
 
   const _MediaCell({
     required this.event,
     required this.client,
     required this.cellSize,
     required this.onTap,
+    required this.onLongPress,
   });
 
   @override
@@ -678,6 +695,7 @@ class _MediaCell extends StatelessWidget {
         : null;
     return GestureDetector(
       onTap: onTap,
+      onLongPress: onLongPress,
       child: ColoredBox(
         color: theme.colorScheme.surfaceContainerHighest,
         child: Stack(