Open the delete-confirmation overlay, or quit when nothing is stored.
Ctrl+D deletes a stored credential, but its priority binding also
intercepts the app-level Ctrl+D=quit. When there's no credential to
delete, fall through to quit rather than swallowing the key (mirroring
the thread selector). app.exit() is used instead of dismiss(), which
would just close the modal silently and re-swallow the key.
