Translate UI and user-facing messages to Ukrainian (#5)

- All frontend pages, labels, notices and errors; html lang=uk, uk-UA money format
- Brand "Assistant System" in the top bar and page title
- Backend error details returned to the UI (auth, orders, receipts,
  cash registers, Checkbox/CRM/NP errors) and CLI output
- Tests updated for the new messages

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/nova_poshta/np_client.py

@@ -21,7 +21,7 @@ class NpTrackingClient:
             return []
         if len(waybill_numbers) > _MAX_DOCUMENTS_PER_REQUEST:
             raise NovaPoshtaError(
-                f"Слишком много ТТН за один запрос: {len(waybill_numbers)} "
+                f"Забагато ТТН за один запит: {len(waybill_numbers)} "
                 f"(максимум {_MAX_DOCUMENTS_PER_REQUEST})"
             )
 
@@ -42,6 +42,6 @@ class NpTrackingClient:
         if not data.get("success"):
             errors = data.get("errors") or []
             message = "; ".join(str(error) for error in errors)
-            raise NovaPoshtaError(f"NP вернул ошибку: {message or 'неизвестная ошибка'}")
+            raise NovaPoshtaError(f"NP повернув помилку: {message or 'невідома помилка'}")
 
         return [TrackingStatusOut.model_validate(item) for item in data.get("data", [])]