Sync Dex relationship priority to Apple and Google Contacts

Make the relationship priority that drives the Priority Map available in Apple/iCloud Contacts and Google Contacts, so it travels with the contact instead of living only in Dex.

Current gap: Priority is a built-in field on every Dex contact (Priority Map and the contact page's priority control are two views of the same field), but nothing carries it outward — two-way CardDAV sync and the Google Contacts integration do not map priority to any vCard field, group or label, so on a phone or in Contacts.app there is no way to tell a Top-priority contact from a Low one.

Desired outcome:

• Dex writes each contact's priority tier out to the synced address book, for example as a label/group per tier or as a dedicated field
• Works for both two-way CardDAV (iCloud/macOS) and Google Contacts
• Priority changes in Dex propagate on the next sync, so the two stay in step
• Ideally opt-in, so people who don't want extra labels on their cards aren't affected