When multiple tabs are open and a token expires, each tab independently
attempts a refresh. With server-side token rotation this causes all but
the first tab to fail, logging the user out.

Add a BroadcastChannel (memos_token_sync) so that when any tab
successfully refreshes, it broadcasts the new token to all other tabs.
Receiving tabs adopt the token in-memory immediately, skipping their own
refresh request and avoiding conflicts with token rotation.

Falls back gracefully when BroadcastChannel is unavailable (e.g. some
privacy modes).