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>
jaredandClaude Opus 4.8
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