dash: attach in place, with popout to full screen

[jtnkminimal]

Reported from dogfooding min dash.

Enter-to-attach should open the session within dash, and allow popping out into full screen after attaching (rather than only a full-screen takeover on attach).

From dash feedback (Slack): https://minimal-dev.slack.com/archives/C091HGA43NC/p1786406103071889?thread_ts=1786387349.174509&cid=C091HGA43NC