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/tests/test_orders_router.py

@@ -88,7 +88,7 @@ def _patch_orders_service(monkeypatch: pytest.MonkeyPatch) -> None:
         session: object, order_id: str, data: object
     ) -> tuple[Order, list[str]] | None:
         if order_id == "locked":
-            raise orders_router.orders_service.OrderEditError("По заказу уже создан чек")
+            raise orders_router.orders_service.OrderEditError("За замовленням уже створено чек")
         if order_id != "1":
             return None
         order = _order(order_id)