The form sends the existing assigneeId on every PATCH even when
the user only changed a different field (e.g. website). If the
vendor's current assignee is a platform admin, every update fails
with "Cannot assign a platform admin as assignee" — even though
no one is trying to change the assignee.

Only run validateAssigneeNotPlatformAdmin when assigneeId is
actually being changed to a different value.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

… onboarding

The vendor mitigation task assigned vendors to whoever triggered
onboarding (authorId). When a platform admin onboards a customer,
all vendors get assigned to the admin. Since platform admins are
filtered from the UI assignee list, vendors appear "Unassigned"
but the backend still has the admin's member ID — which then
blocks all PATCH updates with "Cannot assign a platform admin".

Two fixes:
- Service: only validate assignee when it's actually changing
- Onboarding: skip assignee if author is a platform admin

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>