Replace the swap size text entry with a SpinButton capped to the
selected disk size, consolidate _disk_count_valid and _update_next_button
into a single _is_ready method that checks all preconditions, update the
Next button from on_password_changed so it disables when passwords stop
matching, re-enable the Next button in back_page so navigating back no
longer leaves it stuck disabled, reduce the left panel gap, and fix a
startup crash from mirror_selection firing before swap_entry was created.
ericbsd
708aed30d6