Replace harsh "permanently cancelled" language with friendlier
"The organizer will no longer see you as attending" and rename
buttons from "Cancel attendance" to "Cancel RSVP".
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Allows guests to cancel their RSVP via a DELETE endpoint using their
guestToken. Frontend shows cancel button in RsvpBar and clears local
storage on success. Includes unit tests, integration tests, and E2E spec.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>