Typing "0" into Betalningsvillkor on a customer made the form silently
unsavable: the form schema had min(1) and no error was rendered for the
field, so the user saw nothing happen. 0 days is a real value (betalning
direkt / vid mottagande), and the invoice schema already accepted it.
Customer and supplier forms now validate whole days 0-365 and show the
rule under the field; the API schemas (customer create/update, supplier)
accept 0 the same way; and every `|| 30` fallback that would have turned
a stored 0 back into 30 on edit or create is `?? 30`.
Closes#2070
Claude-Session: https://claude.ai/code/session_01VnConrmMCxJRQ5kfiPPWyy
Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
