feat: remember & restore the last-selected workflow tab

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>

src/services/config_service.py

@@ -11,6 +11,7 @@ DEFAULT_CONFIG = {
     "last_print_dir": "",
     "last_template": "",       # name of the last-selected template
     "last_batch_mode": "full_combo",  # BatchMode value of the last-used mode
+    "last_tab": 0,             # last-selected workflow tab (0=添加印花, 1=AI 穿搭)
     "update_source": "",       # HTTP(S) manifest source (empty = no update check)
     "update_user": "",         # HTTP Basic Auth user (empty = anonymous)
     "update_pass": "",         # HTTP Basic Auth password (empty = anonymous)