Fix ZFS page validation and Next button reliability

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.

install_station/interface_controller.py

@@ -298,4 +298,4 @@ class Interface:
         current_page_widget = cls.page.get_nth_page(cls.page.get_current_page())
         title_text = cls.page.get_tab_label_text(current_page_widget)
         Window.set_title(title_text)
-        # Button.next_button.set_sensitive(True)
+        Button.next_button.set_sensitive(True)