Private sessions — correct times, deposits, and double-booking protection

Four fixes land together for private sessions and events:

- **Bookings are created in your venue's timezone.** When staff created a private-event booking from the admin, the date and time were interpreted in the computer's own timezone — a booking made from a laptop set to another timezone could land an hour or more off. The time you pick is now always read as your venue's local time, including across daylight-saving changes.

- **Deposits charge what the booking shows.** A booking keeps its own deposit amount (set when it was created, and editable by staff). The online deposit payment previously ignored that and recalculated from the event type's settings, so the amount on the pay page could differ from the amount shown on the booking. The booking's own deposit is now the amount charged.

- **Rescheduling checks for clashes.** Moving a private event to a new time, room, or teacher now runs the same availability check as creating one — so a reschedule can no longer silently double-book a room or a teacher who is already teaching.

- **Prices display in your venue's format.** Private-event prices across the admin, the public events pages, and the booking widget now use your venue's own currency and number formatting instead of a fixed Danish format.

Smaller improvements in the same release: the New booking form now flags a group size outside the event type's limits before you submit; the schedule respects each individual booking's "show on schedule" setting; teacher dropdowns now include guest teachers who collaborate with your venue; and changing event-type prices and settings is now reserved for admins and managers.