fix(reminders): RMW race, reliable removal, stable poll interval (N113/N114/N115)

- N113: mutations compute from a local ref kept in sync with server echoes, and
  writes serialize through a promise queue, so rapid add/remove no longer reads
  a stale baseline and clobbers a prior write.
- N114: ReminderMonitor shows each toast once (firedRef) but retries the
  account-data removal on later ticks if it fails (removingRef released on
  error) — a failed removal no longer permanently swallows the reminder.
- N115: the 30s poll interval reads reminders/mDirects via refs and drops them
  from the effect deps, so it's created once instead of resetting its countdown
  on every reminder sync (which could indefinitely defer a near-due reminder).

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

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -390,16 +390,26 @@ function ReminderMonitor() {
   const setToast = useSetAtom(toastQueueAtom);
   const mDirects = useAtomValue(mDirectAtom);
   const firedRef = useRef<Set<string>>(new Set());
+  const removingRef = useRef<Set<string>>(new Set());
+  // Read the latest reminders / DM map via refs so the poll interval below is
+  // created once — not torn down and restarted (which resets its 30s countdown
+  // and can indefinitely defer a near-due reminder) on every reminder sync (N115).
+  const remindersRef = useRef(reminders);
+  remindersRef.current = reminders;
+  const mDirectsRef = useRef(mDirects);
+  mDirectsRef.current = mDirects;
 
   useEffect(() => {
     const check = () => {
       const now = Date.now();
-      reminders.forEach((r) => {
+      remindersRef.current.forEach((r) => {
+        if (r.timestamp > now) return;
         const key = `${r.eventId}-${r.timestamp}`;
-        if (r.timestamp <= now && !firedRef.current.has(key)) {
+        // Show the toast exactly once.
+        if (!firedRef.current.has(key)) {
           firedRef.current.add(key);
           const room = mx.getRoom(r.roomId);
-          const hashPath = mDirects.has(r.roomId)
+          const hashPath = mDirectsRef.current.has(r.roomId)
             ? getDirectRoomPath(r.roomId, r.eventId)
             : getHomeRoomPath(r.roomId, r.eventId);
           setToast({
@@ -410,7 +420,15 @@ function ReminderMonitor() {
             roomId: r.roomId,
             hashPath,
           });
-          removeReminder(r.eventId, r.timestamp);
+        }
+        // Persist the removal, retrying on a later tick if it fails — without
+        // re-showing the toast (N114). The server echo drops it from
+        // `reminders` once the write lands.
+        if (!removingRef.current.has(key)) {
+          removingRef.current.add(key);
+          removeReminder(r.eventId, r.timestamp).catch(() => {
+            removingRef.current.delete(key);
+          });
         }
       });
     };
@@ -425,7 +443,7 @@ function ReminderMonitor() {
       clearInterval(interval);
       document.removeEventListener('visibilitychange', onVisible);
     };
-  }, [mx, reminders, setToast, removeReminder, mDirects]);
+  }, [mx, setToast, removeReminder]);
 
   return null;
 }