Create a simple offline project-price calculator as a single kalkulacka.html file with embedded CSS and JavaScript. It must open by double-clicking in a browser using file://, without a server, account, API, CDN, external fonts, analytics or network requests. Using it must not require AI. Do not create a framework or extra features.

Use an English interface, EUR and amounts excluding VAT. Use a readable light layout, system fonts, visible focus and keyboard controls. Inputs: material and other direct costs, hours of labour, internal hourly cost rate and target margin percentage. Explain that cost rate is not the selling rate and margin is not markup.

Costs N = material + hours × cost rate. Recommended selling price P = N / (1 − margin / 100). Amount above entered costs R = P − N. Never call R net profit. Overheads, taxes and other costs not entered are not included.

Include buttons to load made-up sample figures, clear inputs, export the calculation as JSON and import JSON. The JSON format must have its own identifier and version. Invalid imports and unsupported versions must leave existing inputs unchanged. Check structure, data types and file size. Never render imported data using innerHTML or use eval. Do not save calculations automatically. Clearly tell users to export before closing; do not rely on localStorage.

Accept decimal commas and points. Money permits at most 2 decimal places, hours and percentages 4. Material: 0 to €1 billion; hours: 0 to 1 million; rate: 0 to €1 million/hour. Margin: 0 to less than 100%. Maximum result: €1 trillion. Reject blank inputs, negative numbers, non-numeric and excessive values with clear errors. Clear outdated results on error and restore them when corrected.

Calculate accurately in cents. Round labour cost to cents first, add material, then round selling price to cents. Round an exact half-cent up. Derive R from the displayed figures. Explain that rounding may slightly change the achieved margin. With zero costs, price is €0; a percentage margin at a zero price is undefined. Keep calculation functions separately testable. No discount, VAT, currency selection or other additions.

Test the resulting HTML, not just an isolated formula. Check: material €400, 10 hours × €30, margin 30% gives costs €700, price €1,000 and amount above costs €300. Also test zero margin, zero costs, decimal values, invalid inputs and correction, export → close → reopen → import, malformed JSON and an unsupported version. Check operation without network access in a separate browser context; do not disconnect the whole computer. State the browser actually tested and any unverified parts. Finally show the file and brief instructions for opening it and saving calculations.