feat: #511 - nutrition page minor improvements

[monsieurtanuki]

Impacted file:

• nutrition_page_loaded.dart
  • explicit labels for units
  • removed "energy" line and computed "energy-kj" and "energy-kcal" accordingly
  • unit conversion (e.g. x 1000 for mg from g)
  • new small default nutrient unit repository

[teolemon]

• nb: kj and kcal should not be computed from one another: one field for each
• also, unsure why we need mg / g conversions ? we probably shouldn't try to autoupdate fields on unit change. at the end of the day, the user is responsible for his/her input

[monsieurtanuki]

nb: kj and kcal should not be computed from one another: one field for each

There's no connection between kj and kcal. It's just that sometimes energy-kj was empty but energy was not and energy-unit was kj => instead of displaying an empty energy-kj, I populate it with energy.

also, unsure why we need mg / g conversions ?

If we don't do that, the values are wrong for mg and µg. Just test on a product with vitamin b12.

we probably shouldn't try to autoupdate fields on unit change.

The conversion is just at init time.
There will be another conversion the other way around when saving the nutrients: it looks like we always store the values in grams, and the unit is just there for a better display.