Description of the issue/feature this PR addresses:
Time off can only be rescheduled by opening the leave form and editing its
dates by hand — the calendar is read-only, which is slow for managers who
plan visually.

Current behavior before PR:
Leave events on the Time Off calendar cannot be moved or resized. Changing a
leave's dates requires opening the record and editing request_date_from /
request_date_to (and the AM/PM period or hours) manually.

Desired behavior after PR is merged:
Leave events can be dragged and resized directly on the calendar. The dragged
result is snapped server-side to the leave type's request unit:
- day: move/resize by whole days;
- half_day: endpoints snap to the AM/PM boundary;
- hour: move anywhere, resize on the time grid in hours.
Editing is gated by the new can_reschedule field so approval rules are
honoured; rescheduling an approved leave asks for confirmation and resets it
to "To Approve", and a backend rejection (e.g. an allocation error) reverts
the event.


task-4809240
---
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr