fix(customers): allow 0-day payment terms and say why the field is invalid (#2070) (#2251)

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>

lib/api/__tests__/schemas.test.ts

@@ -704,8 +704,13 @@ describe('CreateCustomerSchema', () => {
     expect(result.success).toBe(false)
   })
 
-  it('rejects zero payment terms', () => {
+  it('accepts zero payment terms (betalning direkt, issue #2070)', () => {
     const result = CreateCustomerSchema.safeParse(validCustomer({ default_payment_terms: 0 }))
+    expect(result.success).toBe(true)
+  })
+
+  it('rejects payment terms above 365 days', () => {
+    const result = CreateCustomerSchema.safeParse(validCustomer({ default_payment_terms: 366 }))
     expect(result.success).toBe(false)
   })