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_service.py

@@ -140,16 +140,16 @@ class TestUpdateOrder:
         assert order.goods == [stored]
 
     def test_total_above_goods_sum_is_rejected(self) -> None:
-        with pytest.raises(OrderEditError, match="больше суммы товаров"):
+        with pytest.raises(OrderEditError, match="більша за суму товарів"):
             _update(_order(), _payload(total_amount="1200.00"))
 
     def test_discount_above_line_sum_is_rejected(self) -> None:
         good = {"name": "Товар", "price": "10.00", "quantity": "1", "discount_amount": "11"}
-        with pytest.raises(OrderEditError, match="скидка больше"):
+        with pytest.raises(OrderEditError, match="знижка більша"):
             _update(_order(), _payload(goods=[good], total_amount="0"))
 
     def test_order_with_receipt_is_locked(self) -> None:
-        with pytest.raises(OrderEditError, match="уже создан чек"):
+        with pytest.raises(OrderEditError, match="уже створено чек"):
             _update(_order(receipt_created_at=datetime.now(UTC)), _payload())
 
     def test_missing_or_deleted_order_returns_none(self) -> None: