feat: add offline queue for kiosk form submissions

Intercepts form submit via fetch and stores failed submissions in
IndexedDB when offline. Replays queued entries on the online event and
on each page load. Shows an offline banner on the form page and a
sync-pending message on the thank-you page.

Service worker bumped to guestbook-v2 to pre-cache offline-queue.js
so the script is available when the kiosk has no network.

templates/thank_you.html

@@ -76,7 +76,11 @@
         <div class="text-center mt-5">
             <div class="alert alert-success py-4" role="alert">
                 <p class="thank-you-message mb-1">Thanks, {{ name }}!</p>
+                {% if offline %}
+                <p class="mb-0">You&rsquo;re currently offline &mdash; your entry has been saved and will sync automatically when reconnected.</p>
+                {% else %}
                 <p class="mb-0">Your visit has been recorded.</p>
+                {% endif %}
             </div>
             <p class="text-muted mt-3">
                 Returning to the form in <span id="countdown">4</span>...
@@ -128,6 +132,8 @@
         })();
     </script>
 
+    <script src="/static/offline-queue.js"></script>
+
     <script>
         if ('serviceWorker' in navigator) {
             window.addEventListener('load', function () {