perf+fix: debounce seq persistence + role changes, null-clear stale peer state

- server: SET_PEER_ROLE now uses the same per-room 500ms debounce as
  SET_CONTROL_MODE (M-4), preventing a buggy/malicious host from
  thrashing the room's UI with rapid promote/demote bursts.
- server: PEER_STATUS relay treats explicit null as 'clear' for
  tabTitle/mediaTitle/currentTime, so the tab-close heartbeat actually
  zeroes out stale state on other peers instead of being silently
  preserved by the clamp fallback.
- background: _persistLastSeq is now trailing-debounced (500ms),
  cutting storage.session IPC writes from one-per-relayed-event to
  one-per-quiet-window in active rooms.
- locales: add missing TOAST_ID_REGENERATED to all 14 non-English
  locales (was blocking locale coverage test on main).
- test: H-1 force-sync demote test now waits out the debounce window
  to reflect real-world UI timing (a host cannot promote, run a
  force-sync, and demote inside 500ms).

extension/locales/fr.json

@@ -109,6 +109,7 @@
   "BTN_REGEN_ID_TOOLTIP": "Régénérer votre identifiant interne et vous reconnecter",
   "REGEN_ID_DESC": "Utilisez cette option si vous rencontrez des erreurs de 'Double identité'.",
   "REGEN_ID_OTHER_ISSUE": "Autre problème? Ouvrez un Issue GitHub",
+  "TOAST_ID_REGENERATED": "Identité régénérée — reconnexion…",
   "LABEL_CONN_STATUS": "Statut de la connexion",
   "LABEL_CONN_STATUS_TOOLTIP": "Statut actuel de la connexion WebSocket",
   "CONN_STATUS_DISCONNECTED": "Déconnecté",