refactor(project): renamings and updated documentation to avoid confusion with the PAI, because this app is just inspired by the PAI.

- Update all README references, descriptions, and terminology
- Reframe app as a science-inspired implementation of NTNU CERG heart rate research
- Replace 'PAI calculation' with 'cardiovascular fitness score' throughout
- Add clear disclaimers regarding non-affiliation with PAI trademark
- Maintain all technical functionality for heart rate score and RHR tracking
