Add edit RSVP modal, plus-one tracking, and unified signup form

Merge RSVP + slot claim into a single form. Add plus_one field to RSVPs.
Add clickable RSVP names that open a modal to edit name/note/plus_one.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/layout.html

@@ -372,6 +372,40 @@
     text-decoration: underline;
   }
   .btn-danger:hover { color: #a00; }
+  .modal-backdrop {
+    position: fixed;
+    inset: 0;
+    background: rgba(0,0,0,0.45);
+    z-index: 90;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 24px;
+  }
+  .modal-card {
+    background: white;
+    border: var(--border-w) solid var(--border);
+    box-shadow: 5px 5px 0 var(--ink);
+    padding: 24px;
+    max-width: 480px;
+    width: 100%;
+    animation: fadeUp 0.2s ease both;
+  }
+  .btn-cancel {
+    background: none;
+    border: none;
+    font-family: 'DM Mono', monospace;
+    font-size: 0.78rem;
+    cursor: pointer;
+    text-decoration: underline;
+    color: #888;
+  }
+  .btn-cancel:hover { color: var(--ink); }
+  .rsvp-name-link {
+    cursor: pointer;
+    text-decoration: none;
+  }
+  .rsvp-name-link:hover { text-decoration: underline; }
   @keyframes fadeUp {
     from { opacity: 0; transform: translateY(16px); }
     to { opacity: 1; transform: translateY(0); }
@@ -438,6 +472,10 @@ document.addEventListener('click', (e) => {
   }
 });
 document.addEventListener('DOMContentLoaded', initEmojiPickers);
+document.body.addEventListener('closeModal', function() {
+  document.getElementById('edit-modal').innerHTML = '';
+});
 </script>
+<div id="edit-modal"></div>
 </body>
 </html>{{end}}