feat: Vue SPA as default interface at root URL

- Route `/` now serves the Vue SPA directly (catch-all `/{req}`)
- Legacy Twig interface moved to `/legacy`
- Vue Router base changed from `/vue/` to `/`

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

templates/menu/menu.html.twig

@@ -2,7 +2,7 @@
     <nav>
         <ul>
             <li class="{{ app.request.get('_route') starts with 'app_manga' ? 'border-l-4 border-green-600' : '' }}">
-                <a href="{{ path('app_manga') }}" class="block pl-4 py-2 flex items-center {{ app.request.get('_route') starts with 'app_manga' ? 'text-green-600 bg-gray-800' : 'hover:bg-gray-700' }}">
+                <a href="{{ path('app_legacy') }}" class="block pl-4 py-2 flex items-center {{ app.request.get('_route') starts with 'app_manga' ? 'text-green-600 bg-gray-800' : 'hover:bg-gray-700' }}">
                     <i class="fas fa-book mr-2"></i>
                     <span>Mangas</span>
                 </a>