Fix call-cluster bugs #11, #4a, #4b

#11: adding a 3rd person to a 1:1 call then having one participant close
the app disconnected the call for everyone. leaveMeeting() ended a DM room
for ALL peers on any leave; now it only tears down when <2 people remain,
otherwise it falls through to the normal peer-left path (call continues).

#4b: after someone left a call they never reappeared under "Add people".
meeting-peer-left cleaned meetPeers/tiles but not meetPeerUids/meetNames,
so the departed uid stayed in hereUids and was filtered out. Now deleted.

#4a: a guest who enabled mic/cam on the pre-join screen had to re-tap after
being admitted — the choices were applied on a blind 900ms timer that fired
while still in the lobby. Now applied in the meeting-joined handler, after
admission + media connect.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/signaling.js

@@ -406,8 +406,11 @@ async function leaveMeeting(ws) {
   if (!peers) { if (leaverId) CHAT.broadcastPresence(leaverId); return; }
   try { await require('./calls').finalizeTranscript(room, ws._meetingUserId); } catch (_) {} // save THIS user's transcript
   peers.delete(pid);
-  // 1:1 call: when either party leaves, end it for everyone (a DM call has no "remaining" call).
-  if (roomToDmCall.has(room)) {
+  // 1:1 call: end it for everyone ONLY when fewer than two people would remain. A DM call that had
+  // extra people ADDED (via "Add people") is effectively a group now — one participant closing their
+  // app must NOT hang up the call for the rest (#11). We only tear the whole thing down when ≤1 person
+  // is left (nobody to talk to). With 2+ remaining we fall through to the normal peer-left path below.
+  if (roomToDmCall.has(room) && peers.size < 2) {
     const others = [...peers.values()].map((p) => p.ws && p.ws._meetingUserId).filter(Boolean);
     for (const [, p] of peers) { if (p.ws.readyState === 1) { try { p.ws.send(JSON.stringify({ type: 'meeting-ended' })); } catch (_) {} p.ws._meetingRoom = null; } }
     await persistCallHistory(room); // #7: log the call BEFORE endCallByRoom clears roomToDmCall/roomToGroupCall