Fix macOS PiP window not resizing when switching videos

Switching videos during active PiP left the window at the old video's
size with white space around the new content, for two reasons:

- MPV reports the new video's width and height as separate property
  events while the old values are kept across switches, so a transient
  mixed size (new width with old height) could reach the PiP capture
  path and AVKit sized the window from that frame. Coalesce the PiP
  capture-size/aspect update with a short debounce so only settled
  dimensions are used.

- On macOS nothing told AVKit about the new aspect: unlike iOS, the
  bridge never flushed the sample buffer renderer on an aspect change
  during active PiP, and AVKit doesn't resize the macOS PiP window on
  its own. Flush the renderer, reset the format description, and
  resize the PiP window to the new aspect ratio explicitly.

Yattee/Services/Player/MPVBackend.swift

@@ -127,6 +127,8 @@ final class MPVBackend: PlayerBackend {
     // Video dimensions for aspect ratio detection
     private var videoWidth: Int = 0
     private var videoHeight: Int = 0
+    // Coalesces separate width/height property events into one PiP size update
+    private var pipVideoSizeUpdateTask: Task<Void, Never>?
     // Cached video FPS to avoid sync fetch on main thread
     private var containerFps: Double = 0
     // Cached cache state to avoid sync fetch on main thread
@@ -1732,17 +1734,28 @@ extension MPVBackend: MPVClientDelegate {
         LoggingService.shared.debug("MPV: Video size detected: \(videoWidth)x\(videoHeight)", category: .mpv)
         delegate?.backend(self, didUpdateVideoSize: videoWidth, height: videoHeight)
 
-        // Update PiP bridge with video aspect ratio for proper window sizing
+        // Update PiP capture dimensions and aspect ratio, coalesced.
+        // Width and height arrive as separate MPV property events and the old
+        // values are kept across video switches, so right after a switch one of
+        // them can still be stale (e.g. new width paired with old height).
+        // Debounce so capture buffers and the PiP window never see that
+        // transient mixed size.
         #if os(iOS) || os(macOS)
-        let aspectRatio = CGFloat(videoWidth) / CGFloat(videoHeight)
-        pipBridge?.updateVideoAspectRatio(aspectRatio)
-        #endif
-
-        // Update render view with video content dimensions for accurate PiP capture
-        // (avoids capturing letterbox/pillarbox black bars)
-        #if os(iOS) || os(macOS)
-        renderView?.videoContentWidth = videoWidth
-        renderView?.videoContentHeight = videoHeight
+        pipVideoSizeUpdateTask?.cancel()
+        pipVideoSizeUpdateTask = Task { [weak self] in
+            try? await Task.sleep(for: .milliseconds(100))
+            guard let self, !Task.isCancelled else { return }
+            let width = self.videoWidth
+            let height = self.videoHeight
+            guard width > 0, height > 0 else { return }
+            // Content dimensions must update before the aspect ratio: on macOS
+            // the aspect update flushes the PiP sample buffer during active PiP,
+            // and a stale-sized frame captured afterwards would make AVKit
+            // re-read the old dimensions.
+            self.renderView?.videoContentWidth = width
+            self.renderView?.videoContentHeight = height
+            self.pipBridge?.updateVideoAspectRatio(CGFloat(width) / CGFloat(height))
+        }
         #endif
 
         // Update render view with video FPS for display link frame rate matching