[Likes][Mentions] Post control items order can change

This isn't a bug, but an annoyance.

People have reported multiple times that the items swap places. This happens whenever the enabled extensions order is resolved, I believe the order falls back to randomness, hence why mentions can sometimes load before likes and other times the other way around.

I see two possible solutions:
* Explicitly set priorities for these items in JS side so that they're always ordered in the same way.
* Change the extension boot order resolving to fallback to something, like the package name.

Start in the JavaScript side that renders the post control items and trace how the extension boot order is resolved. Compare the two proposed approaches—explicit item priorities or a deterministic package-name fallback—and confirm that mentions consistently follow likes without disrupting other extensions.

Written by the indexing model from the issue text.