feat: Add Picture-in-Picture floating video + fix local track visibility

- New PictureInPicture component: draggable floating window that shows
  active stream when navigating away from voice channel, with LIVE badge,
  snap-to-edge, click-to-navigate-back
- Fix VoiceUser to check actual track presence instead of boolean flags
  for video rendering
- Add LocalTrackPublished/LocalTrackUnpublished events to useLiveKit
  so local screen share and camera show immediately in VoiceGrid
- Add pipCollapsed state to uiStore

packages/web/src/components/voice/VoiceUser.js

@@ -44,7 +44,7 @@ export function VoiceUser({ participant, large }) {
         }
         audioEl.muted = isDeafened;
     }, [isDeafened, outputVolume, perUserVolume]);
-    const hasVideo = participant.isCameraOn || participant.isScreenSharing;
+    const hasVideo = !!(participant.videoTrack || participant.screenTrack);
     const isLocal = participant.isLocal;
     // Volume context menu
     const [volumeMenu, setVolumeMenu] = useState(null);