feat: setting to choose the default follow-up mode (Queue or Steer)

[andybergon]

Since #11673, a message sent during a running turn queues by default on web and desktop (steer is the arrow button), while mobile still steers immediately on send. There is no way to choose what plain Enter does, and the default is inconsistent across clients.

Add a setting for the default follow-up mode, Queue or Steer, applied consistently on web, desktop, and mobile. People who want a follow-up to interrupt the running turn keep Steer as the default; people who want it to wait keep Queue.

Follow-up to #6684, which added both modes but no default preference.

[juliusmarminge]

Resolved by #11964 — Settings → General → Follow-up behavior (Queue or Steer) on web/desktop.