Booking Pages: Allow "Appts Can Be Booked Every" interval to start at a custom time offset (e.g., :30 past the hour)

Currently, the "Appts can be booked every" setting on booking pages snaps to standard clock increments from the top of the hour. This means if your availability starts at :30 past the hour, switching to "every hour" intervals will not display any slots at all, forcing users to use 30-minute intervals instead — which displays twice as many slots as are actually bookable.

Specific use case: I run 45-minute appointments starting at 2:30 PM, with a 15-minute buffer between sessions for notes and reset time. This gives me a clean one-appointment-per-hour schedule starting at :30. This is a common real-world scenario for coaches, therapists, and consultants who cannot always begin their day on the hour.

The requested improvement would allow users to either set a custom start offset for the slot interval (e.g., "every 60 minutes, starting at :30"), or introduce a "first available slot" time field that anchors the interval calculation. This would allow the booking page display to accurately reflect true availability without showing unbookable slots to clients.