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/utils/screenShare.ts

@@ -18,7 +18,7 @@ export interface OverdriveOptions {
 
 export interface ScreenShareBuildResult {
   capture: { width: number; height: number; frameRate: number };
-  publish: { videoCodec: 'h264'; videoEncoding: { maxBitrate: number; maxFramerate: number }; simulcast: false };
+  publish: { videoCodec: 'vp9'; videoEncoding: { maxBitrate: number; maxFramerate: number }; simulcast: false };
   overdrive: OverdriveOptions;
   contentHint: 'motion' | 'detail';
 }
@@ -63,7 +63,7 @@ export function buildScreenShareOptions(config: ScreenShareConfig): ScreenShareB
   return {
     capture: { width, height, frameRate: fps },
     publish: {
-      videoCodec: 'h264',
+      videoCodec: 'vp9',
       videoEncoding: { maxBitrate, maxFramerate: fps },
       simulcast: false,
     },
@@ -99,14 +99,19 @@ export async function applyOverdrive(
     if (!sender) return;
 
     const params = sender.getParameters();
-    if (!params.encodings?.[0]) return;
+    if (!params.encodings?.length) return;
 
-    params.encodings[0].maxBitrate = options.maxBitrate;
-    params.encodings[0].maxFramerate = options.maxFramerate;
-    params.encodings[0].networkPriority = 'high';
+    // Target the highest-quality layer. With simulcast, encodings[0] is the
+    // lowest layer; our overdrive must hit the top layer so the custom bitrate
+    // slider controls the full-resolution stream, not the quarter-res one.
+    // For non-simulcast tracks (single encoding), length - 1 === 0.
+    const idx = params.encodings.length - 1;
+    params.encodings[idx]!.maxBitrate = options.maxBitrate;
+    params.encodings[idx]!.maxFramerate = options.maxFramerate;
+    params.encodings[idx]!.networkPriority = 'high';
     (params as any).degradationPreference = options.degradationPreference;
     if (options.minBitrate > 0) {
-      (params.encodings[0] as any).minBitrate = options.minBitrate;
+      (params.encodings[idx] as any).minBitrate = options.minBitrate;
     }
 
     await sender.setParameters(params);
@@ -134,9 +139,9 @@ export async function startScreenShare(room: Room): Promise<boolean> {
       // @ts-ignore — LiveKit accepts frameRate at capture level
       frameRate: opts.capture.frameRate,
     }, {
-      videoCodec: 'h264',
+      videoCodec: opts.publish.videoCodec,
       videoEncoding: opts.publish.videoEncoding,
-      simulcast: false,
+      simulcast: opts.publish.simulcast,
     } as any);
 
     if (!track) {