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>

assets/vue/app/router/index.js

@@ -168,6 +168,6 @@ const routes = [
 ];
 
 export const router = createRouter({
-    history: createWebHistory('/vue/'),
+    history: createWebHistory('/'),
     routes
 });