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/background.js

@@ -132,8 +132,14 @@ chrome.runtime.onConnect.addListener((port) => {
     }
 });
 
+let _persistLastSeqTimer = null;
 function _persistLastSeq() {
-    if (storageInitialized) chrome.storage.session.set({ lastSeqBySender });
+    if (!storageInitialized) return;
+    if (_persistLastSeqTimer) clearTimeout(_persistLastSeqTimer);
+    _persistLastSeqTimer = setTimeout(() => {
+        _persistLastSeqTimer = null;
+        chrome.storage.session.set({ lastSeqBySender });
+    }, 500);
 }
 
 // --- Boot Sequence Lock ---