Prevent users from scheduling events in the past by mistake

Newdle currently doesn't seem to differentiate time slots in the past and in the future.

It could be useful to indicate to the user that they are selecting a day in the past, for which it would be too late to schedule anything. For example:

- Display a warning that the timeslot is in the past
- Grey-out options in the past, but letting the user select them
- Combination of the two previous points
- Grey-out options in the past, not letting the user select them (if proposing slots in the past is not considered a supported feature).

Thanks!

No file, test, or entry point is named. Start by locating the time-slot selection UI and its existing date-handling tests, then confirm which treatment of past slots maintainers want. Done means the agreed warning or disabled-state behavior is implemented and covered by tests.

Written by the indexing model from the issue text.