Soften RSVP cancellation dialog wording

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>

frontend/src/components/RsvpBar.vue

@@ -25,7 +25,7 @@
             type="button"
             @click="$emit('cancel')"
           >
-            Cancel attendance
+            Cancel RSVP
           </button>
         </Transition>
       </div>