Convert CSV (spreadsheet) to TCX

A CSV with a lap-index column is more useful than it might look: it's the one lap representation FitFileHub's own schema supports, and it's exactly what TCX is built to represent well. This tool reads each row using the documented column names — timestamp, lat, lon, elevation_m, plus optional hr_bpm, cadence_rpm, power_w, and lap columns — with tolerance for common alias headers if your spreadsheet wasn't built to that exact spec. Where CSV only carries lap boundaries as a per-point index, TCX has real lap structure: this tool groups points by that index and writes genuine TCX Lap elements, each with its own point sequence, giving back the structured activity a CSV can only approximate on its own. Heart rate, cadence, and power all map onto TCX's native trackpoint fields in full; only temperature has no home in TCX, so that column is dropped if present in the source. Timestamps come across at TCX's second-resolution precision. If your CSV has no lap column at all, the output is simply a single-lap TCX — still a fully valid, device-ready activity file. Everything runs locally in your browser, in a background worker for larger files, so spreadsheet data of any size converts without ever being uploaded to anyone.