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

configs/airootfs/root/configurator

@@ -908,7 +908,13 @@ select_installation() {
       full_disk_only=false
     fi
 
-    install_mode_form
+    # With no free-space option to offer, a full-disk install is the only
+    # mode, so skip the picker and go straight to the overwrite confirm.
+    if $full_disk_only; then
+      install_mode="Full disk install"
+    else
+      install_mode_form
+    fi
 
     case "$install_mode" in
       "Full disk install")
@@ -916,6 +922,9 @@ select_installation() {
           install_target="full_disk"
           return 0
         fi
+        # When the picker was skipped, declining the overwrite means the user
+        # wants a different disk.
+        $full_disk_only && disk_form
         ;;
       "Free space install"*)
         if run_partition_decide; then