feat: animate queue sidebar expansion and collapse

Checks/system-playback.swift

@@ -299,10 +299,12 @@ struct SystemPlaybackChecks {
         model.attachWindow(window)
         let videoFrame = window.frame
         model.showsQueue = true
+        until("queue opening settles") { !model.isQueueTransitioning }
         require(abs(window.frame.width - videoFrame.width - 260) < 0.01 &&
                 abs(window.frame.height - videoFrame.height) < 0.01,
                 "opening queue grows window without shrinking video")
         model.showsQueue = false
+        until("queue closing settles") { !model.isQueueTransitioning }
         require(abs(window.frame.width - videoFrame.width) < 0.01 &&
                 abs(window.frame.height - videoFrame.height) < 0.01,
                 "closing queue restores video-only window size")
@@ -327,6 +329,7 @@ struct SystemPlaybackChecks {
         }
         until("open starts authoritative playback") { model.canTogglePlayback && model.state.status == PLAYER_PLAYING }
         model.showsQueue = true
+        until("queue opens before live resize") { !model.isQueueTransitioning }
         for delta in [NSSize(width: 160, height: 0), NSSize(width: 0, height: 90),
                       NSSize(width: -100, height: 0), NSSize(width: 0, height: -60)] {
             let proposal = NSSize(width: window.frame.width + delta.width, height: window.frame.height + delta.height)
@@ -343,6 +346,7 @@ struct SystemPlaybackChecks {
                     "aspect-locked queue window follows the dragged axis")
         }
         model.showsQueue = false
+        until("queue closes after live resize") { !model.isQueueTransitioning }
         require(center.playbackState == .playing, "playing publishes macOS playback ownership")
         requirePlaybackPower(system: true, display: true, "local video keeps the Mac and display awake")
         var bufferingState = model.state