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_receipts_service.py

@@ -77,17 +77,17 @@ class TestResolveAmounts:
         assert amounts.cod_kopecks == 100000
 
     def test_explicit_prepayment_must_match_cod(self) -> None:
-        with pytest.raises(svc.ReceiptValidationError, match="≠ наложка"):
+        with pytest.raises(svc.ReceiptValidationError, match="≠ післяплата"):
             svc.resolve_amounts(_order(np_cod_amount_kopecks=100000), 10000)
 
     @pytest.mark.parametrize(
         ("overrides", "message"),
         [
-            ({"waybill_number": None}, "нет ТТН"),
-            ({"np_cod_amount_kopecks": None}, "контроля оплаты"),
-            ({"np_status_code": "9"}, "не в пути"),
-            ({"np_cod_amount_kopecks": 130000}, "больше суммы заказа"),
-            ({"is_deleted": True}, "удалён"),
+            ({"waybill_number": None}, "немає ТТН"),
+            ({"np_cod_amount_kopecks": None}, "контролю оплати"),
+            ({"np_status_code": "9"}, "не в дорозі"),
+            ({"np_cod_amount_kopecks": 130000}, "більша за суму замовлення"),
+            ({"is_deleted": True}, "видалено"),
         ],
     )
     def test_rejects(self, overrides: dict[str, Any], message: str) -> None:
@@ -226,7 +226,7 @@ class TestRequestReceipts:
         )
 
         assert result.created == []
-        assert "Касса не определена" in result.errors[order.id]
+        assert "Касу не визначено" in result.errors[order.id]
 
 
 class FakeSession:
@@ -289,7 +289,7 @@ class TestCreateEttn:
         await svc.create_ettn_for_receipt(FakeSession(order, register, receipt), client, receipt.id)
 
         assert receipt.status == ReceiptStatus.FAILED
-        assert "уже привязана" in (receipt.error or "")
+        assert "вже прив'язана" in (receipt.error or "")
         assert order.receipt_created_at is None
 
     async def test_unavailable_keeps_pending_then_reconciles(self) -> None: