fix(reminders): make cancel optimistic, no contradictory error

The shared account-data store removes optimistically with no rollback, so the
reminder row vanished the instant Cancel was clicked. Showing a "could not
cancel" error beside the already-gone row (and it reappearing on next sync) was
self-contradictory. Match the removeBookmark convention: fire-and-forget
optimistic removal, no inline error. Drops the now-moot cancelling busy-guard
and uses a collision-safe React key for same-minute custom reminders.

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

src/app/features/room/message/RemindMeDialog.tsx

@@ -62,7 +62,6 @@ export function RemindMeDialog({ roomId, eventId, previewText, onClose }: Remind
   const [busy, setBusy] = useState(false);
   const [error, setError] = useState<string | null>(null);
   const [customOpen, setCustomOpen] = useState(false);
-  const [cancelling, setCancelling] = useState<Set<number>>(new Set());
   const def = useMemo(() => defaultCustomDate(), []);
   const [dateValue, setDateValue] = useState(() => toLocalDate(def));
   const [timeValue, setTimeValue] = useState(() => toLocalTime(def));
@@ -83,21 +82,13 @@ export function RemindMeDialog({ roomId, eventId, previewText, onClose }: Remind
     if (customOpen) dateInputRef.current?.focus();
   }, [customOpen]);
 
-  const handleCancelExisting = async (timestamp: number) => {
-    if (cancelling.has(timestamp)) return;
-    setCancelling((prev) => new Set(prev).add(timestamp));
-    setError(null);
-    try {
-      await removeReminder(eventId, timestamp);
-    } catch {
-      setError('Could not cancel reminder. Try again.');
-    } finally {
-      setCancelling((prev) => {
-        const next = new Set(prev);
-        next.delete(timestamp);
-        return next;
-      });
-    }
+  const handleCancelExisting = (timestamp: number) => {
+    // Optimistic, matching removeBookmark: the shared account-data store drops
+    // the reminder locally at once (no rollback) and re-syncs from the server.
+    // We deliberately show no inline error — the store has no rollback path, so a
+    // failed write simply reappears on the next sync rather than leaving a stale
+    // "couldn't cancel" message beside an already-vanished row.
+    removeReminder(eventId, timestamp).catch(() => undefined);
   };
 
   const commit = async (timestamp: number) => {
@@ -191,8 +182,10 @@ export function RemindMeDialog({ roomId, eventId, previewText, onClose }: Remind
                   <Text size="L400" priority="300" style={{ paddingLeft: config.space.S200 }}>
                     {existing.length === 1 ? 'Reminder set' : 'Reminders set'}
                   </Text>
-                  {existing.map((r) => (
-                    <Box key={r.timestamp} alignItems="Center" gap="200">
+                  {existing.map((r, idx) => (
+                    // Composite key: two custom reminders on one message can share
+                    // a minute-precision timestamp; index keeps React keys unique.
+                    <Box key={`${r.timestamp}-${idx}`} alignItems="Center" gap="200">
                       <Icon src={Icons.Clock} size="100" style={{ flexShrink: 0 }} />
                       <Text size="T200" style={{ flexGrow: 1, minWidth: 0 }} truncate>
                         {formatFriendlyDateTime(r.timestamp)}
@@ -202,7 +195,6 @@ export function RemindMeDialog({ roomId, eventId, previewText, onClose }: Remind
                         radii="300"
                         variant="SurfaceVariant"
                         fill="None"
-                        disabled={cancelling.has(r.timestamp)}
                         onClick={() => handleCancelExisting(r.timestamp)}
                         aria-label={`Cancel reminder for ${formatFriendlyDateTime(r.timestamp)}`}
                       >