Heart rate tracking and weight fixes

- Record resting heart rate on its own or alongside a blood pressure reading
- Heart rate now appears in recent entries, the home screen, and the blood pressure chart
- Fixed the home screen showing weight in the wrong unit (e.g. converted kg value labelled as lbs)
- Fixed weight validation so limits apply correctly in kg, lbs, and stones
- Fixed being unable to save weights entered in stones
- Clearer waist chart colours (waist in blue, waist-to-height ratio in green)
