fix: enforce write phase completion gate and wire done endpoint

- GET /write now checks all groups are written/skipped before showing
  the completion screen; incomplete sessions are redirected to the first
  draft group
- POST /write/done now accepts form data (not JSON) and redirects to
  /export; wired up from the completion screen via a <form> POST button
- phase5.html extra_scripts block wrapped in {% if group %} to prevent
  Jinja errors when group is None on the completion screen

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

services/travel-memories/app/templates/phase5.html

@@ -7,7 +7,11 @@
   </div>
 
   {% if not group %}
-  <div class="alert alert-success">All groups written or skipped. <a href="/export?album_id={{ album_id }}" class="link">Continue to export →</a></div>
+  <div class="alert alert-success mb-4">All groups written or skipped.</div>
+  <form method="post" action="/write/done">
+    <input type="hidden" name="album_id" value="{{ album_id }}">
+    <button type="submit" class="btn btn-primary">Export →</button>
+  </form>
   {% else %}
   <div class="flex gap-4">
 
@@ -101,6 +105,7 @@
 {% endblock %}
 
 {% block extra_scripts %}
+{% if group %}
 <script>
 (function() {
   var albumId = {{ album_id | tojson }};
@@ -193,4 +198,5 @@
   }
 })();
 </script>
+{% endif %}
 {% endblock %}