fix(host-control-mode): audit findings — host dedup demotion, episode-lobby freeze, +cleanups

Findings recovered from the multi-agent audit (verification phase was cut off by a
usage limit; verified inline against the code):

HIGH:
- Host was demoted on peerId dedup (fast reconnect / second tab): the dedup path
  removes the old socket and the kicked socket's 'disconnect' both ran the
  host-leave fallback before the same peerId re-joined → room silently unlocked on
  every host network blip. Now skip the fallback while a join for that peerId is in
  flight (peerJoinLocks). Regression test added. (A long real disconnect still
  falls back — that's the deferred host-grace EC-10.)
- Episode auto-advance froze a gated guest: in host-only the guest's EPISODE_LOBBY
  is dropped server-side, but the guest still self-paused (PAUSE_FOR_LOBBY) waiting
  for readies that never came → 60s freeze. A host-only guest now skips creating a
  lobby (the host drives episode sync).

LOW / cleanup:
- GET_HCM_STRINGS could return a raw key name ("HCM_DIALOG_TITLE") if the locale
  dictionary failed to load (getMessage returns the key on miss) → omit it so
  content keeps its English fallback.
- hcmReset now also clears the snap-back cooldown + buffering grace, so a stale
  cooldown can't swallow the first snap-back after a room/host change.
- Popup play/pause labels reset on lock too (not just unlock), so a button can't
  freeze on "Playing…" when host-only activates mid-click.
- Fix a stale comment (heartbeat now carries 'desynced').

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

extension/content.js

@@ -390,6 +390,8 @@
         const wasDesynced = hcmDesynced;
         hcmDesynced = false;
         hcmDeferredSnapPending = false;
+        hcmSnapBackCooldownUntil = 0; // don't let a stale cooldown swallow the next snap-back
+        hcmBufferingUntil = 0;
         hcmRemoveDialog();
         hcmRemoveBadge();
         // If we were desynced, notify background so it stops reporting us as