fix: generate video thumbnails locally when server provides none

- Show gray placeholder immediately when no thumbnailSource (instead of
  perpetual spinner)
- Generate thumbnails using AVAssetImageGenerator — downloads the video
  and extracts the first frame
- Cache generated thumbnails in the shared NSCache for reuse

Author: beezly

Mactrix/Views/ChatView/MessageVideoView.swift

@@ -10,6 +10,7 @@ struct MessageVideoView: View {
 
     @State private var fileHandle: MediaFileHandle?
     @State private var video: AVPlayer?
+    @State private var generatedThumbnail: Image?
 
     var aspectRatio: CGFloat? {
         guard let info = content.info,
@@ -47,14 +48,60 @@ struct MessageVideoView: View {
         }
     }
 
+    private func generateThumbnail() async {
+        guard let client = appState.matrixClient?.client else { return }
+
+        let cacheKey = NSString(string: "thumb:" + content.source.url())
+        if let cached = MatrixClient.imageCache.object(forKey: cacheKey) {
+            generatedThumbnail = Image(nsImage: cached)
+            return
+        }
+
+        do {
+            let handle = try await client.getMediaFile(
+                mediaSource: content.source,
+                filename: content.filename,
+                mimeType: content.info?.mimetype ?? "",
+                useCache: true,
+                tempDir: NSTemporaryDirectory()
+            )
+            fileHandle = handle
+            let path = try handle.path()
+            let url = URL(filePath: path, directoryHint: .notDirectory)
+
+            let asset = AVURLAsset(url: url)
+            let generator = AVAssetImageGenerator(asset: asset)
+            generator.appliesPreferredTrackTransform = true
+            generator.maximumSize = CGSize(width: 600, height: 600)
+
+            let cgImage = try await generator.image(at: .zero).image
+            let nsImage = NSImage(cgImage: cgImage, size: NSSize(width: cgImage.width, height: cgImage.height))
+            MatrixClient.imageCache.setObject(nsImage, forKey: cacheKey)
+            generatedThumbnail = Image(nsImage: nsImage)
+        } catch {
+            Logger.viewCycle.error("Failed to generate video thumbnail: \(error)")
+        }
+    }
+
+    @ViewBuilder
+    var thumbnailView: some View {
+        if let thumbnailSource = content.info?.thumbnailSource {
+            MatrixImageView(mediaSource: thumbnailSource, mimeType: content.info?.thumbnailInfo?.mimetype)
+        } else if let generatedThumbnail {
+            generatedThumbnail.resizable().scaledToFit()
+        } else {
+            Rectangle().fill(Color.gray.opacity(0.3))
+        }
+    }
+
     var body: some View {
         VStack {
             if let video {
                 TimelineVideoPlayer(videoPlayer: video)
                     .cornerRadius(6)
             } else {
                 Button(action: { Task { await loadVideo() } }) {
-                    MatrixImageView(mediaSource: content.info?.thumbnailSource, mimeType: content.info?.thumbnailInfo?.mimetype)
+                    thumbnailView
                         .overlay {
                             Image(systemName: "play.fill")
                                 .resizable()
@@ -73,5 +120,10 @@ struct MessageVideoView: View {
         }
         .frame(maxHeight: maxHeight)
         .aspectRatio(aspectRatio, contentMode: .fit)
+        .task(id: content.source.url(), priority: .utility) {
+            if content.info?.thumbnailSource == nil {
+                await generateThumbnail()
+            }
+        }
     }
 }