Introduces db/seeds/quantities.rb with a default "Body weight" quantity
(user_id nil = system default), mirroring the pattern used by seeds/units.rb.
Wires it into db/seeds.rb so it runs on db:seed.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
* replace variable names with Hash to avoid invalid Ruby identifiers
* export all values as single-quoted to avoid string interpolation and
treating BigDecimal numbers as Float
* #truncate table instead of #delete_all to avoid foreing_key
constraints errors
Closes#56
