web: e2e coverage for user offboarding

Track browser coverage for scheduled user offboarding (`authentik/enterprise/lifecycle/offboarding/`).

Scope:
- Schedule, list, cancel, and reschedule an offboarding.
- Reject a schedule in the past.
- Hide self-offboarding while allowing regular service accounts.
- Bulk-cancel pending offboardings.
- Execute a scheduled deactivation and a scheduled deletion, asserting the audit events.

Supersedes the stale branch behind #24369, whose tests were written against an earlier iteration of the offboarding UI.