fix: chat-open scroll regression, SFU screen reliability, unanswered calls, selected-chat border

- REGRESSION (More#1): removed overflow-anchor:none (it stopped the view from staying
  at the bottom as images settle) + renderThread re-asserts scroll-to-bottom after late
  content, unless a 'New messages' divider is shown → chats open at the latest message.
- #9: dynacast off — a screen-share layer was being paused unless a camera track was
  also flowing (blank / only-with-camera / slow). Now every published track flows.
- More#7/#9: 1:1 calls track 'answered'; unanswered calls auto-end after ~40s (caller
  no longer stuck ringing) and post 'Missed call' instead of a duration; answered calls
  show duration from the answer time. meeting-ended reason 'unanswered' → 'No answer'.
- More#6: selected chat now has a thin yellow (brand) boundary.

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

server/signaling.js

@@ -87,7 +87,7 @@ function handle(ws, m, req) {
       // …and tell existing peers a newcomer arrived.
       for (const [, p] of peers) { if (p.ws.readyState === 1) p.ws.send(JSON.stringify({ type: 'meeting-peer-joined', peerId, name, avatar, uid: ws._meetingUserId || null })); }
       peers.set(peerId, { ws, name, avatar, uid: ws._meetingUserId || null });
-      if (ws._meetingUserId) CHAT.broadcastPresence(ws._meetingUserId); // now in a call → update contacts live
+      if (ws._meetingUserId) { try { require('./calls').markDmAnswered(room, ws._meetingUserId); } catch (_) {} CHAT.broadcastPresence(ws._meetingUserId); } // #9: callee joined → mark 1:1 answered
       const tsubs = transcriptSubs.get(room); if (tsubs && tsubs.size > 0) ws.send(JSON.stringify({ type: 'meeting-transcribe-state', active: true })); // catch up: already transcribing
       break;
     }