feat: smart codec defaults + mid-stream codec switching

- Gaming mode defaults to H.264 (NVENC hardware encoding, zero CPU)
- Text mode defaults to VP9 (better compression, CPU is free)
- Changing mode auto-updates codec to smart default
- User can manually override codec via UI pills
- Changing codec mid-stream auto-restarts the stream (~1s interruption)

packages/web/src/hooks/useLiveKit.ts

@@ -32,6 +32,7 @@ import {
 import { getMediaStreamTrack } from '../utils/livekitInternals';
 
 let _activeRoom: Room | null = null;
+let _publishedScreenShareCodec: 'vp9' | 'h264' | null = null;
 
 export function getActiveRoom(): Room | null {
   return _activeRoom;
@@ -608,8 +609,21 @@ export function useLiveKit() {
     const updateActiveTracks = async () => {
       if (isScreenSharing) {
         const opts = buildScreenShareOptions(screenShareConfig);
+
+        // Codec changed mid-stream — must restart (codec is baked into SDP negotiation)
+        if (_publishedScreenShareCodec && _publishedScreenShareCodec !== opts.publish.videoCodec) {
+          _publishedScreenShareCodec = null;
+          await stopScreenShare(room);
+          setTimeout(() => startScreenShare(room).catch(() => {}), 200);
+          return;
+        }
+
         const screenPub = room.localParticipant.getTrackPublications().find(p => p.source === Track.Source.ScreenShare);
         if (screenPub?.videoTrack) {
+          // Track the published codec for mid-stream change detection
+          if (!_publishedScreenShareCodec) {
+            _publishedScreenShareCodec = opts.publish.videoCodec;
+          }
           const mediaTrack = getMediaStreamTrack(screenPub.videoTrack);
           if (mediaTrack) {
             if (opts.capture.width > 0 && opts.capture.height > 0) {
@@ -625,6 +639,9 @@ export function useLiveKit() {
           resolveNativeOverdrive(mediaTrack ?? null, screenShareConfig, opts);
           await applyOverdrive(room, Track.Source.ScreenShare, opts.overdrive);
         }
+      } else {
+        // Screen share stopped — clear published codec tracker
+        _publishedScreenShareCodec = null;
       }
       if (isCameraOn) { await applyOverdrive(room, Track.Source.Camera, CAMERA_OVERDRIVE); }
     };

packages/web/src/stores/voiceStore.ts

@@ -146,7 +146,7 @@ export const useVoiceStore = create<VoiceState>()(
       inputDeviceId: 'default',
       outputDeviceId: 'default',
       focusedParticipantId: null,
-      screenShareConfig: { height: 720, fps: 60, mode: 'gaming', codec: 'vp9', customBitrateKbps: null, shareAudio: !isElectron() },
+      screenShareConfig: { height: 720, fps: 60, mode: 'gaming', codec: 'h264', customBitrateKbps: null, shareAudio: !isElectron() },
       participantVolumes: new Map(),
       setParticipantVolume: (userId, volume) => {
         set((state) => {
@@ -326,9 +326,14 @@ export const useVoiceStore = create<VoiceState>()(
       toggleScreenShare: () => set((state) => ({ isScreenSharing: !state.isScreenSharing })),
 
       setFocusedParticipant: (id) => set({ focusedParticipantId: id }),
-      setScreenShareConfig: (config) => set((state) => ({
-        screenShareConfig: { ...state.screenShareConfig, ...config },
-      })),
+      setScreenShareConfig: (config) => set((state) => {
+        const merged = { ...state.screenShareConfig, ...config };
+        // When mode changes (without an explicit codec override), auto-set the smart default
+        if (config.mode && !config.codec) {
+          merged.codec = config.mode === 'gaming' ? 'h264' : 'vp9';
+        }
+        return { screenShareConfig: merged };
+      }),
       noiseSuppression: true,
       echoCancellation: true,
       autoGainControl: true,