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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>