Fix host stream freeze when a guest joins an in-progress party

A joining member's player always called loadVideo(itemId, 0, false),
hardcoding a resume position of 0. Since a party's Emby stream is one
shared transcode session (PartyStreamService, keyed by partyId alone),
a newcomer's hls.js requesting segments from position 0 against a
session that already progressed well past that forces Emby to
seek/restart the shared transcode - stalling playback for every
member, host included, right as the newcomer joins.

Seed the initial resume position from the host's last-reported
position (already available in party.members) instead, so the first
segment request lands near where the shared session already is.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

client/src/views/player/Player.ts

@@ -1260,7 +1260,20 @@ function renderVideoPlayer(root: HTMLElement, partyId: string, party: PartyPubli
 
     loadMarkers(party.itemId); // not awaited - see onItemChanged's own comment
     await loadTrackOptions(party.itemId);
-    loadVideo(party.itemId, 0, false);
+    // Unlike onItemChanged's own loadVideo(..., 0, ...) - genuinely correct
+    // there since a fresh item has no position yet - mounting mid-party
+    // (a joining guest, or the host's own page reload) must NOT hardcode 0:
+    // the party's shared Emby transcode (PartyStreamService) is keyed by
+    // partyId alone, one PlaySessionId for every member, so a hls.js
+    // instance requesting segments from 0 against a session that already
+    // progressed well past that forces Emby to seek/restart the shared
+    // transcode - stalling it for every other member, host included, right
+    // as the newcomer joins. Seeding from the host's last-reported position
+    // (already in party.members, sent to every mount) lands the first
+    // segment request near where the shared session already is instead.
+    const hostMember = party.members.find((m) => m.isHost);
+    const initialResumeSeconds = hostMember ? hostMember.positionTicks / TICKS_PER_SECOND : 0;
+    loadVideo(party.itemId, initialResumeSeconds, false);
 
     conn = connectParty(partyId, {
       onMembers: renderMembers,