fix: ML shipping config check + improved payload + actionable error messages

- Add get_shipping_preferences to meli_service.py
- Add check_meli_shipping_config to validate ME2 adoption before publishing
- Include local_pick_up and free_shipping in item payload
- Translate ME2/mode errors to actionable Spanish messages
- Check shipping config in both validate_items and publish_items

pos/services/meli_service.py

@@ -183,6 +183,9 @@ class MeliService:
     def get_category_attributes(self, category_id: str) -> list:
         return self._request("GET", f"/categories/{category_id}/attributes")
 
+    def get_shipping_preferences(self, user_id: str) -> dict:
+        return self._request("GET", f"/users/{user_id}/shipping_preferences")
+
     # ─── Orders ──────────────────────────────────────────────────────────
 
     def get_orders(