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/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)
+        }
+    }
+}