Add the gallery timeline item rendered as a placeholder

Introduces the GalleryRoomTimelineItem and its typed GalleryItem model (reusing the existing per-message content types), builds it in the RoomTimelineItemFactory and routes it through RoomTimelineItemView. For now it renders as a placeholder; the grid/list media views and the full-screen preview land in follow-up PRs.

Co-authored-by: svetoslav.mitov <svetoslav.mitov@sportinno.com>

ElementX/Sources/Screens/Timeline/View/Threads/TimelineThreadSummaryView.swift

@@ -82,6 +82,12 @@ struct TimelineThreadSummaryView: View {
                                plainBody: L10n.commonSharedLocation,
                                formattedBody: nil,
                                numberOfReplies: numberOfReplies)
+                case .gallery(let content):
+                    ThreadView(senderID: senderID,
+                               sender: sender,
+                               plainBody: content.caption ?? content.body,
+                               formattedBody: content.formattedCaption,
+                               numberOfReplies: numberOfReplies)
                 }
             case .poll(let question):
                 ThreadView(senderID: senderID,