♻️(frontend) refactor prejoin screen for room context flexibility

Decouple prejoin components from conference context to enable different
behaviors when inside vs outside room environments. Components can now
evolve independently with lighter coupling.

Refactor layout structure to prepare for upcoming speaker selector
introduction. This decoupling allows for more flexible component
evolution and cleaner architecture.

src/frontend/src/locales/nl/rooms.json

@@ -8,6 +8,9 @@
     "back": "Sluit weer bij de vergadering aan"
   },
   "join": {
+    "selectDevice": {
+      "loading": "Bezig met laden…"
+    },
     "videoinput": {
       "choose": "Selecteer camera",
       "disable": "Camera uitschakelen",