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/receipts.py

@@ -60,7 +60,7 @@ async def list_receipts(session: SessionDep, order_id: str | None = None) -> lis
 async def get_receipt(receipt_id: uuid.UUID, session: SessionDep) -> ReceiptOut:
     receipt = await session.get(Receipt, receipt_id)
     if receipt is None:
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Чек не найден")
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Чек не знайдено")
     return ReceiptOut.from_receipt(receipt)
 
 
@@ -83,5 +83,5 @@ async def cancel_receipt(
             status_code=status.HTTP_502_BAD_GATEWAY, detail=f"Checkbox: {exc}"
         ) from exc
     if receipt is None:
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Чек не найден")
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Чек не знайдено")
     return ReceiptOut.from_receipt(receipt)