slides: add /screensaver route for the shopfloor slide surface

TVDashboard hardcoded surface=lobby, so only the lobby display was reachable.
Read the surface from route meta (/screensaver -> shopfloor) or a ?surface=
query, defaulting to lobby. Adds a /screensaver toplevel route so the shopfloor
screensaver surface can be displayed on a kiosk.

plugins/slides/frontend/routes.js

@@ -13,6 +13,12 @@ export const toplevel = [
     path: '/tv',
     name: 'tv',
     component: () => import('./views/TVDashboard.vue'),
-    meta: { plugin: 'slides' }
+    meta: { plugin: 'slides', surface: 'lobby' }
+  },
+  {
+    path: '/screensaver',
+    name: 'screensaver',
+    component: () => import('./views/TVDashboard.vue'),
+    meta: { plugin: 'slides', surface: 'shopfloor' }
   }
 ]