how to convert Calc spreadsheet into mobile-friendly web application, so that it can be used for calculation online? No need to show all cells and formulas, only two fields are needed: input field (were user enters the number) and result field.
A mobile-friendly web application never ever uses any spreadsheet at all. There are web applications able to edit uploaded spreadsheets, such as https://www.collaboraonline.com/ or Google spreadsheets.