Handle Clerk membership-limit-exceeded errors gracefully on invite flows

[ChesterSchendel]

Context

Production Clerk is on the free plan, which caps org membership at 20 members per org (per Clerk's current pricing). When we exceed this on either:

• Admin provisioning (/api/admin/clients POST)
• Team Lead inviting designers (/dashboard/team invite form, calling organization.inviteMember)

Clerk's API returns an error. The current code surfaces the raw error string with no specific handling.

What to build

1. Detect Clerk's specific "membership limit exceeded" error code/message in both routes.
2. Surface a user-friendly message: e.g., "This team has reached its member limit. Please contact your Drawbackwards account manager to upgrade." on the Lead's invite UI; a similar admin-facing message on /admin/clients.
3. Document the cap in /hq decisions or features for the team's awareness.

Priority

Block before any real client onboards with >20 designers. Not blocking right now (Drawbackwards internal org is well under 20).