fix: use LiveKit track.attach() for adaptive stream and switch screen share to VP9 single-layer

Camera was stuck at 180p because our custom <video> rendering bypassed
LiveKit's adaptive stream observer. Replaced manual srcObject binding
with track.attach()/detach() in VoiceUser, StreamTile, and PictureInPicture
so the SFU receives viewport dimensions and forwards the correct
H.264 simulcast layer.

Screen share VP9 SVC with L3T3 spatial layers failed because hardware
VP9 encoders (NVENC, QSV, VCE) don't support spatial scalability —
Chrome silently degrades to L1T1. Reverted to VP9 single-layer
(simulcast: false, no scalabilityMode). Also targets encodings[length-1]
in applyOverdrive() for correct simulcast layer targeting.

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

@@ -34,16 +34,19 @@ export function VoiceUser({ tile, large }: VoiceUserProps) {
     return () => tile.videoTrack?.removeEventListener('ended', onEnded);
   }, [tile.videoTrack]);
 
-  // Attach Video
+  // Attach Video — use LiveKit's track.attach() to register the element
+  // with the adaptive stream observer (enables SFU layer switching by viewport size)
   useEffect(() => {
     const videoEl = videoRef.current;
     if (!videoEl) return;
-    if (activeVideoTrack) {
-      videoEl.srcObject = new MediaStream([activeVideoTrack]);
+    const lkTrack = tile.lkVideoTrack;
+    if (lkTrack) {
+      lkTrack.attach(videoEl);
+      return () => { lkTrack.detach(videoEl); };
     } else {
       videoEl.srcObject = null;
     }
-  }, [activeVideoTrack]);
+  }, [tile.lkVideoTrack]);
 
   // Context Menu
   const [volumeMenu, setVolumeMenu] = useState<{