Default booking link on the booking page (admin-set, users can only disable)

Every account should always have a default booking link on its booking page that can be shared from prospecting tools (email, SMS, info pages, outreach). Admins own that default link; individual users should not be able to replace it — the only control available to them is to disable it.

Why it matters: prospecting tools need one predictable link that always resolves, instead of depending on whatever link a user happened to create or edit. It also stops team members accidentally breaking outbound booking flows by changing the shared link.

Desired behaviour:
• A default booking link exists on every booking page and works everywhere prospecting tools reference a booking link.
• Admins can set/change the default link.
• Regular users cannot change it — they can only turn it off.