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>

config/routes.yaml

@@ -5,7 +5,7 @@ controllers:
     type: attribute
 
 vue_app:
-    path: /vue/{req}
+    path: /{req}
     controller: Symfony\Bundle\FrameworkBundle\Controller\TemplateController
     defaults:
         template: 'vue/index.html.twig'