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/Services/Timeline/TimelineItems/Items/Messages/GalleryRoomTimelineItem.swift

@@ -0,0 +1,31 @@
+//
+// 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 UIKit
+
+nonisolated struct GalleryRoomTimelineItem: EventBasedMessageTimelineItemProtocol, Equatable {
+    let id: TimelineItemIdentifier
+    let timestamp: Date
+    let isOutgoing: Bool
+    let isEditable: Bool
+    let canBeRepliedTo: Bool
+    var shouldBoost = false
+    
+    let sender: TimelineItemSender
+    
+    let content: GalleryRoomTimelineItemContent
+    
+    var properties = RoomTimelineItemProperties()
+    
+    var body: String {
+        content.caption ?? content.body
+    }
+    
+    var contentType: EventBasedMessageTimelineItemContentType {
+        .gallery(content)
+    }
+}

ElementX/Sources/Services/Timeline/TimelineItems/Items/Messages/GalleryRoomTimelineItemContent.swift

@@ -0,0 +1,145 @@
+//
+// 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 Foundation
+import UniformTypeIdentifiers
+
+nonisolated struct GalleryRoomTimelineItemContent: Hashable {
+    let body: String
+    var caption: String?
+    var formattedCaption: AttributedString?
+    /// The original textual representation of the formatted caption directly from the event (usually HTML code)
+    var formattedCaptionHTMLString: String?
+    let items: [GalleryItem]
+}
+
+/// Identifies a single attachment within a gallery message.
+nonisolated struct GalleryItemID: Hashable {
+    /// The unique ID of the parent gallery timeline item.
+    let timelineItemUniqueID: TimelineItemIdentifier.UniqueID
+    /// The item's position within the gallery.
+    let mediaIndex: Int
+}
+
+/// A single attachment of a gallery message. The SDK hands back the same content types it uses for
+/// individual messages, so we reuse them here rather than duplicating their fields.
+nonisolated enum GalleryItem: Hashable, Identifiable {
+    case image(id: GalleryItemID, ImageRoomTimelineItemContent)
+    case video(id: GalleryItemID, VideoRoomTimelineItemContent)
+    case audio(id: GalleryItemID, AudioRoomTimelineItemContent)
+    case file(id: GalleryItemID, FileRoomTimelineItemContent)
+    /// An item of an unknown type. It has no media source, so there's nothing to preview.
+    case other(id: GalleryItemID, filename: String)
+    
+    var id: GalleryItemID {
+        switch self {
+        case .image(let id, _), .video(let id, _), .audio(let id, _), .file(let id, _), .other(let id, _):
+            id
+        }
+    }
+    
+    var isImage: Bool {
+        if case .image = self {
+            true
+        } else {
+            false
+        }
+    }
+    
+    var isVideo: Bool {
+        if case .video = self {
+            true
+        } else {
+            false
+        }
+    }
+    
+    var isAudio: Bool {
+        if case .audio = self {
+            true
+        } else {
+            false
+        }
+    }
+    
+    var isFile: Bool {
+        if case .file = self {
+            true
+        } else {
+            false
+        }
+    }
+    
+    var filename: String {
+        switch self {
+        case .image(_, let content): content.filename
+        case .video(_, let content): content.filename
+        case .audio(_, let content): content.filename
+        case .file(_, let content): content.filename
+        case .other(_, let filename): filename
+        }
+    }
+    
+    var mediaSource: MediaSourceProxy? {
+        switch self {
+        case .image(_, let content): content.imageInfo.source
+        case .video(_, let content): content.videoInfo.source
+        case .audio(_, let content): content.source
+        case .file(_, let content): content.source
+        case .other: nil
+        }
+    }
+    
+    var thumbnailSource: MediaSourceProxy? {
+        switch self {
+        case .image(_, let content): content.thumbnailInfo?.source
+        case .video(_, let content): content.thumbnailInfo?.source
+        case .file(_, let content): content.thumbnailSource
+        case .audio, .other: nil
+        }
+    }
+    
+    var size: CGSize? {
+        switch self {
+        case .image(_, let content): content.imageInfo.size
+        case .video(_, let content): content.videoInfo.size
+        case .audio, .file, .other: nil
+        }
+    }
+    
+    var blurhash: String? {
+        switch self {
+        case .image(_, let content): content.blurhash
+        case .video(_, let content): content.blurhash
+        case .audio, .file, .other: nil
+        }
+    }
+    
+    var duration: TimeInterval? {
+        switch self {
+        case .video(_, let content): content.videoInfo.duration
+        case .audio(_, let content): content.duration
+        case .image, .file, .other: nil
+        }
+    }
+    
+    var contentType: UTType? {
+        switch self {
+        case .image(_, let content): content.contentType
+        case .video(_, let content): content.contentType
+        case .audio(_, let content): content.contentType
+        case .file(_, let content): content.contentType
+        case .other: nil
+        }
+    }
+    
+    /// Whether the item can render a visible thumbnail. Images always can (their media source
+    /// IS the image); for everything else we need an explicit thumbnail source.
+    var hasThumbnail: Bool {
+        isImage || thumbnailSource != nil
+    }
+}