feat(voice): add cachedUser carry-forward to ParticipantInfo

Extends ParticipantInfo with cachedUser: User | null. In updateParticipants,
snapshots the previous participant list and carries forward the cached User
object when spaceStore member lookup fails (user navigated to a different
space). Local user falls back to authStore. homeUserId is now derived from
cachedUser when the direct member lookup misses. Updates EMPTY_PARTICIPANT
in PictureInPicture.tsx.

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

@@ -14,6 +14,7 @@ const EMPTY_PARTICIPANT: ParticipantInfo = {
   isMuted: false, isDeafened: false, isCameraOn: false, isScreenSharing: false,
   isLocal: false, audioTrack: null, videoTrack: null, screenTrack: null,
   screenAudioTrack: null, lkVideoTrack: null, lkScreenTrack: null,
+  cachedUser: null,
 };
 
 const PIP_WIDTH = 320;