No Join button anywhere on the organization page (for the org or its rings)

**What happens:** When a user lands on an organization page, there is no Join button anywhere — not for the organization itself, not for any of the rings listed on the page. The user has no way to join anything from this page. To join a ring, they have to navigate away (Nearby, search, etc.) and find that same ring somewhere else. **Expected:** the ring cards shown on the organization page should each have a Join button so users can join a ring directly from the org page they're already on. (The organization itself may not need a separate Join action — what users typically want is to join one of its rings.) **Where:** organization detail page (`/organizations/{slug}`) — specifically the section that lists the org's rings. **Repro:** 1. Sign in as a user who isn't a member of any ring. 2. Navigate to an organization page that lists rings. 3. Try to join one of those rings from this page. 4. Observe: no Join button on the ring cards, no Join button for the org. Dead end — the user has to leave the page to find a Join action elsewhere. **Related context (user-reported):** new users have trouble telling what an organization is vs. a ring. When they reach an org page expecting some way to participate and find no Join action, the page feels broken or empty. Adding the Join button to the ring cards here would let them act on their intent immediately.