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/Replies/TimelineReplyView.swift

@@ -129,6 +129,13 @@ struct TimelineReplyView: View {
                                   plainBody: L10n.commonSharedLocation,
                                   formattedBody: nil,
                                   icon: .init(kind: .icon(\.locationPin)))
+                    case .gallery(let content):
+                        ReplyView(sender: sender,
+                                  plainBody: content.caption ?? content.body,
+                                  formattedBody: content.formattedCaption,
+                                  icon: content.items.first?.thumbnailSource.map { .init(kind: .mediaSource($0)) }
+                                      ?? content.items.first?.mediaSource.map { .init(kind: .mediaSource($0)) }
+                                      ?? .init(kind: .icon(\.attachment)))
                     }
                 case .poll(let question):
                     ReplyView(sender: sender,

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,

ElementX/Sources/Screens/Timeline/View/TimelineItemViews/GalleryRoomTimelineView.swift

@@ -0,0 +1,22 @@
+//
+// Copyright 2026 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+import Compound
+import SwiftUI
+
+struct GalleryRoomTimelineView: View {
+    let timelineItem: GalleryRoomTimelineItem
+    
+    var body: some View {
+        TimelineStyler(timelineItem: timelineItem) {
+            // Placeholder rendering — the grid/list media views land in a follow-up.
+            FormattedBodyText(text: timelineItem.content.caption ?? timelineItem.content.body,
+                              trailingReservedSize: timelineItem.trailingReservedSize,
+                              boostFontSize: timelineItem.shouldBoost)
+        }
+    }
+}