Support for all-day reminders (date-only, no time)

[zhixianio]

When creating a reminder with only a date (e.g., --due 2026-01-16 or --due today), remindctl sets the time to midnight (00:00). However, Apple Reminders supports "all-day" reminders that have a date but no specific time.

Current behavior:

remindctl add "Test reminder" --due 2026-01-16
# Result: Jan 16, 2026 at 0:00

Expected behavior:

A way to create all-day reminders without a specific time, matching the native Apple Reminders behavior when you set a due date without checking "Add Time".

Suggested solutions:

1. Add an --all-day flag: remindctl add "Test" --due 2026-01-16 --all-day
2. Or make date-only inputs default to all-day (no time), requiring explicit time for timed reminders: --due "2026-01-16 09:00"
   Use case:
   When adding daily tasks/todos, users often want them associated with a day rather than a specific time. The current midnight default can be confusing, especially across timezones.

[psculthorpe]

Adding support here — I use date-only reminders as my default workflow and the midnight time causes confusion. Would love either an --all-day flag or making date-only the default when no time is specified.

[kylejfrost]

Submitted with #24.

[LuisUrrutia]

I would love to have this.