Date picking — decouple from react-day-picker v10 selection state
Dates couldn't be picked (same in notion-page-clone) — the controlled mode/selected/onSelect path broke under react-day-picker v10. The calendar now handles clicks via onDayClick and drives the selected-day + range highlight purely from `modifiers`, both fed by our own value, so it no longer relies on rdp's internal selection state machine.
Slices touched
- notion-database — date picking via onDayClick (rdp-v10-proof)