🚸(room) prevent user from misclicking out of a meeting

now clicking on the header homepage link asks for confirmation when on
the route page.

this is quick and dirty, using browser confirm ui, and not making a
difference between join page and conference page, but it'll do for now

src/frontend/src/locales/de/rooms.json

@@ -5,5 +5,6 @@
     "joinLabel": "",
     "micLabel": "",
     "userLabel": ""
-  }
+  },
+  "leaveRoomPrompt": ""
 }