fix: add CSRF token to participant logout form

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/templates/participant/dashboard.html

@@ -41,6 +41,7 @@
 
     <section>
         <form method="POST" action="{{ url_for('participant.logout') }}">
+            <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
             <button type="submit">Logout</button>
         </form>
     </section>