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/api/v1/orders.py

@@ -50,7 +50,7 @@ async def update_order(
     except orders_service.OrderEditError as exc:
         raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail=str(exc)) from exc
     if result is None:
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Заказ не найден")
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Замовлення не знайдено")
 
     order, changed = result
     if changed:
@@ -75,7 +75,7 @@ async def delete_order(
 ) -> None:
     order = await orders_service.delete_order(session, order_id)
     if order is None:
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Заказ не найден")
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Замовлення не знайдено")
 
     await audit.record(
         session,