Add RSVP support and Open Graph meta tags

People can now RSVP without claiming a slot. OG tags show
event title, date/time/location, and headcount in link previews.

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

templates/layout.html

@@ -4,6 +4,7 @@
 <meta charset="UTF-8">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>{{block "title" .}}picnic{{end}}</title>
+{{block "meta" .}}{{end}}
 <link href="https://fonts.googleapis.com/css2?family=Bricolage+Grotesque:opsz,wght@12..96,400;12..96,700;12..96,800&family=DM+Mono:wght@400;500&display=swap" rel="stylesheet">
 <script src="https://unpkg.com/htmx.org@2.0.4"></script>
 <script src="https://unpkg.com/htmx-ext-sse@2.2.2/sse.js"></script>
@@ -319,6 +320,12 @@
     background: var(--yellow);
     border-color: var(--border);
   }
+  .rsvp-list {
+    display: flex;
+    flex-wrap: wrap;
+    gap: 8px;
+    margin-bottom: 40px;
+  }
   .btn-danger {
     background: none;
     border: none;