feat: add loading spinner when starting a new game

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

templates/homepage/index.html.twig

@@ -29,10 +29,17 @@
         </div>
     {% else %}
         <div class="game-start-container">
-            <form method="post" action="{{ path('app_game_start') }}">
+            <form method="post" action="{{ path('app_game_start') }}" id="start-form">
                 <input type="hidden" name="_token" value="{{ csrf_token('game_start') }}">
                 <button type="submit" class="btn btn-primary btn-start">Commencer une partie</button>
             </form>
+            <div class="start-loader" id="start-loader"></div>
+            <script>
+                document.getElementById('start-form').addEventListener('submit', function () {
+                    this.style.display = 'none';
+                    document.getElementById('start-loader').style.display = 'block';
+                });
+            </script>
         </div>
     {% endif %}
 {% endblock %}