Support sending gallery messages behind a feature flag

Adds the ability to send a gallery message (multiple attachments in one message)
via the media upload preview, wired through the timeline controller/proxy to the
SDK's sendGallery. Gated by a `galleryEnabled` developer flag (default off),
matching Element X Android — with the flag off, attachments are sent as
individual messages as before.

Extends the community PR #5601, rebased onto develop. Tracking:
element-hq/element-meta#3201

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

ElementX/Sources/Screens/MediaUploadPreviewScreen/View/MediaUploadPreviewScreen.swift

@@ -6,6 +6,7 @@
 // Please see LICENSE files in the repository root for full details.
 //
 
+import AVFoundation
 import Combine
 import Compound
 import GameController
@@ -35,17 +36,7 @@ struct MediaUploadPreviewScreen: View {
         mainContent
             .id(context.viewState.mediaURLs)
             .ignoresSafeArea(edges: [.horizontal])
-            .safeAreaInset(edge: .top) {
-                if context.viewState.mediaURLs.count > 1 {
-                    Text(L10n.screenMediaUploadPreviewItemCount(currentIndex + 1, context.viewState.mediaURLs.count))
-                        .font(.compound.bodyMD)
-                        .foregroundColor(.compound.textPrimary)
-                        .padding(.vertical, 4)
-                        .padding(.horizontal, 8)
-                        .background(.compound.bgBadgeDefault)
-                        .clipShape(.capsule)
-                }
-            }
+            .overlay(alignment: .top) { galleryBadge }
             .safeAreaInset(edge: .bottom, spacing: 0) {
                 composer
                     .padding(.horizontal, 12)
@@ -74,6 +65,20 @@ struct MediaUploadPreviewScreen: View {
             }
     }
     
+    @ViewBuilder
+    private var galleryBadge: some View {
+        if context.viewState.mediaURLs.count > 1 {
+            Text(L10n.screenMediaUploadPreviewItemCount(currentIndex + 1, context.viewState.mediaURLs.count))
+                .font(.compound.bodySMSemibold)
+                .foregroundStyle(.compound.textPrimary)
+                .padding(.horizontal, 12)
+                .padding(.vertical, 6)
+                .background(Color.compound.bgCanvasDefault.opacity(0.85),
+                            in: Capsule())
+                .padding(.top, 12)
+        }
+    }
+    
     @ViewBuilder
     private var mainContent: some View {
         if ProcessInfo.processInfo.isiOSAppOnMac {
@@ -81,6 +86,9 @@ struct MediaUploadPreviewScreen: View {
                 .font(.compound.headingMD)
                 .foregroundColor(.compound.textSecondary)
                 .frame(maxWidth: .infinity, maxHeight: .infinity)
+        } else if context.viewState.mediaURLs.count > 1 {
+            UploadMediaPeekCarousel(mediaURLs: context.viewState.mediaURLs,
+                                    currentIndex: $currentIndex)
         } else {
             PreviewView(mediaURLs: context.viewState.mediaURLs,
                         title: context.viewState.title,
@@ -214,6 +222,133 @@ struct MediaUploadPreviewScreen: View {
     }
 }
 
+private struct UploadMediaPeekCarousel: View {
+    let mediaURLs: [URL]
+    @Binding var currentIndex: Int
+    
+    @State private var scrolledID: Int?
+    
+    var body: some View {
+        ScrollView(.horizontal, showsIndicators: false) {
+            LazyHStack(spacing: 8) {
+                ForEach(Array(mediaURLs.enumerated()), id: \.offset) { index, url in
+                    UploadMediaThumbnail(url: url)
+                        .containerRelativeFrame(.horizontal)
+                        .id(index)
+                }
+            }
+            .scrollTargetLayout()
+        }
+        .contentMargins(.horizontal, 24, for: .scrollContent)
+        .scrollTargetBehavior(.viewAligned)
+        .scrollPosition(id: $scrolledID)
+        .onAppear {
+            if scrolledID == nil {
+                scrolledID = currentIndex
+            }
+        }
+        .onChange(of: scrolledID) { _, newValue in
+            if let newValue, newValue != currentIndex {
+                currentIndex = newValue
+            }
+        }
+    }
+}
+
+private struct UploadMediaThumbnail: View {
+    let url: URL
+    
+    private var contentType: UTType? {
+        UTType(filenameExtension: url.pathExtension)
+    }
+    
+    private var isImageOrVideo: Bool {
+        guard let contentType else { return false }
+        return contentType.conforms(to: .image) || contentType.conforms(to: .movie) || contentType.conforms(to: .audiovisualContent)
+    }
+    
+    var body: some View {
+        Group {
+            if isImageOrVideo {
+                UploadMediaImageThumbnail(url: url)
+            } else {
+                UploadMediaFilePreview(url: url, title: url.lastPathComponent)
+            }
+        }
+        .clipShape(RoundedRectangle(cornerRadius: 8))
+    }
+}
+
+private struct UploadMediaImageThumbnail: View {
+    let url: URL
+    @State private var image: UIImage?
+    
+    var body: some View {
+        ZStack {
+            Color.black
+            if let image {
+                Image(uiImage: image)
+                    .resizable()
+                    .scaledToFit()
+            } else {
+                ProgressView().tint(.white)
+            }
+        }
+        .task(id: url) { await load() }
+    }
+    
+    private func load() async {
+        if let image = UIImage(contentsOfFile: url.path(percentEncoded: false)) {
+            self.image = image
+            return
+        }
+        
+        // Fall back to a video frame thumbnail.
+        let asset = AVURLAsset(url: url)
+        let generator = AVAssetImageGenerator(asset: asset)
+        generator.appliesPreferredTrackTransform = true
+        do {
+            let cgImage = try await generator.image(at: .zero).image
+            image = UIImage(cgImage: cgImage)
+        } catch {
+            image = nil
+        }
+    }
+}
+
+private struct UploadMediaFilePreview: UIViewControllerRepresentable {
+    let url: URL
+    let title: String
+    
+    func makeUIViewController(context: Context) -> QLPreviewController {
+        let controller = QLPreviewController()
+        controller.dataSource = context.coordinator
+        return controller
+    }
+    
+    func updateUIViewController(_ uiViewController: QLPreviewController, context: Context) { }
+    
+    func makeCoordinator() -> Coordinator {
+        Coordinator(url: url, title: title)
+    }
+    
+    final class Coordinator: NSObject, QLPreviewControllerDataSource {
+        private let item: PreviewItem
+        
+        init(url: URL, title: String) {
+            item = PreviewItem(previewItemURL: url, previewItemTitle: title)
+        }
+        
+        func numberOfPreviewItems(in controller: QLPreviewController) -> Int {
+            1
+        }
+        
+        func previewController(_ controller: QLPreviewController, previewItemAt index: Int) -> QLPreviewItem {
+            item
+        }
+    }
+}
+
 private struct PreviewView: UIViewControllerRepresentable {
     let mediaURLs: [URL]
     let title: String?
@@ -396,7 +531,8 @@ struct MediaUploadPreviewScreen_Previews: PreviewProvider, TestablePreview {
                                                              mediaUploadingPreprocessor: MediaUploadingPreprocessor(appSettings: .volatile()),
                                                              timelineController: TimelineControllerMock(.init()),
                                                              clientProxy: ClientProxyMock(.init()),
-                                                             userIndicatorController: UserIndicatorControllerMock())
+                                                             userIndicatorController: UserIndicatorControllerMock(),
+                                                             galleryEnabled: true)
     static var previews: some View {
         ElementNavigationStack {
             MediaUploadPreviewScreen(context: viewModel.context)