Only show the mode picker when free space is also an option

Restore the pre-OEM behavior: a disk with no unallocated free space has just
one viable mode, so the wizard skips the mode picker and goes straight to the
overwrite confirmation. The picker appears only when a free-space install is
also on offer.

Also give the harness's first-boot SSH bootstrap more headroom (360s): an
encrypted OEM first boot re-keys LUKS and rebuilds the UKI before sshd
settles, which can exceed 180s under parallel VM load.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/omarchy-iso-test

@@ -872,7 +872,7 @@ EOF
   capture_console "success-first-boot-05-bootstrap-command"
   press ret
 
-  wait_for_ssh 180 "failure-first-boot-ssh-timeout"
+  wait_for_ssh 360 "failure-first-boot-ssh-timeout"
   capture_console "success-first-boot-06-bootstrap-complete"
 
   kill "$HTTP_PID" 2>/dev/null || true