RSVP on a non-member event errors out — should prompt to join the group first

**What happens:** When a user who is NOT a member of a group tries to RSVP to an event hosted by that group, the action fails with an error. The user is given no actionable path forward. **Expected:** instead of an error, the user should be prompted: "You need to join the group to RSVP. Want to join?" with a Join button. After joining (one tap), the original RSVP should either auto-complete or the user should be returned to the event with the RSVP buttons now enabled. **Where:** event detail page, RSVP button. Affects any event whose hosting group the current user has not yet joined. Likely affects both Going / Interested / Maybe RSVP states. **Repro:** 1. Sign in as a user. 2. Navigate to a group you are NOT a member of. 3. Open one of its events. 4. Tap the RSVP button (Going / Interested / etc). 5. Observe: error message, no path forward. 6. Expected: prompt to join the group, then auto-RSVP (or re-enable the RSVP action after join).