lemmy-ui: set meta.platforms to nodejs.meta.platforms

lemmy-ui should run everywhere that nodejs does.

pkgs/servers/web-apps/lemmy/ui.nix

···
     homepage = "https://join-lemmy.org/";
     license = licenses.agpl3Only;
     maintainers = with maintainers; [ happysalada billewanick ];
-    platforms = platforms.linux;
+    inherit (nodejs.meta) platforms;
   };
 }