The backend already returns is_available:false for past slots, but a
slot can lapse after the list is loaded. Disable any slot whose start
(unix seconds) is before now in the time grid, so a just-passed slot
can't be clicked. The grid disabling makes a SendAppo-level guard
redundant — a past slot can no longer be selected.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>