Add last_tab to DEFAULT_CONFIG; main_window restores it in _restore_preferences
(only enabled/implemented tabs 0/1; setCurrentIndex before connecting the persist
slot to avoid an echo write) and persists via _on_tab_persist on currentChanged.
Mirrors the existing last_template/last_batch_mode pattern. Document the key in
docs/02 §9. Offscreen-verified (restore 0/1, disabled 2 -> 0, persist); suite green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>