fix(suppliers): stop requiring standardkonto that was never meant to be required (#1636)

* fix(suppliers): stop requiring standardkonto that was never meant to be required

The supplier form initializes every optional field to '' and sent them
as-is, while CreateSupplierSchema validates default_expense_account
with the 4-digit account rule behind .optional(): an empty string is a
present string, so saving a supplier with the field untouched failed
with "Kontonummer måste vara 4 siffror" even though the field carries
no required mark (reported by Björn with a screen recording; the edit
page failed the same way for any supplier without a default account).

Schemas now own the normalization, split by verb: on create '' becomes
undefined (key dropped, column NULL), on update '' becomes null,
because update routes pass fields straight into .update() where
undefined means "leave unchanged" and clearing must actually write
NULL. Email gets the same treatment and the form's old client-side
email strip is removed; stripping empty strings client-side would
break exactly the clear path.

The free-text Standardkonto input is replaced with the shared
AccountCombobox (browsable list filtered to cost classes 4-7, the same
rule the agent-path expenseAccountField enforces), with the selected
account name shown under the field and a clear button when set.
Standardkonto itself stays optional: it only prefills supplier-invoice
lines and the ledger-context suggestion covers the empty case.

Verified end to end against the running app: saving a supplier without
a default account succeeds on the update path, and the combobox
search/select/clear cycle works inside the create dialog.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(api-spec): render preprocess pipes by output side, required-ness by undefined-acceptance

The minimal Zod-to-JSON-schema walker described every pipe by its input
side. For .transform() that is right (the caller sends the input), but
z.preprocess() is the mirror image: the callable sits on the input side,
so the supplier schemas' new empty-string normalization rendered email
and default_expense_account as required untyped fields in the OpenAPI
spec and the generated accounted-api skill. Describe the output side
when the input is a transform.

Required-ness now derives from schema.safeParse(undefined) instead of a
top-level discriminator check: a field may be omitted exactly when the
schema accepts undefined. Besides the preprocess pipes, this corrects
several fields the old check misrendered as required (z.unknown()
bodies, union-with-empty-string settings fields, preprocessed
personal_number), so the regenerated skill references only flip
required to optional where runtime validation already allowed omission.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/api/__tests__/schemas.test.ts

@@ -743,6 +743,24 @@ describe('CreateSupplierSchema', () => {
     const result = CreateSupplierSchema.safeParse(validSupplier({ default_expense_account: '6200' }))
     expect(result.success).toBe(true)
   })
+
+  // The web form submits untouched optional inputs as '' (Björn 2026-08-17:
+  // saving with the field left blank failed "Kontonummer måste vara 4 siffror").
+  it('treats empty-string expense account as absent', () => {
+    const result = CreateSupplierSchema.safeParse(validSupplier({ default_expense_account: '' }))
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.default_expense_account).toBeUndefined()
+    }
+  })
+
+  it('treats empty-string email as absent', () => {
+    const result = CreateSupplierSchema.safeParse(validSupplier({ email: '' }))
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.email).toBeUndefined()
+    }
+  })
 })
 
 // ============================================================
@@ -2206,6 +2224,39 @@ describe('UpdateSupplierSchema', () => {
     expect(result.success).toBe(true)
   })
 
+  // Update routes pass fields straight into .update(), where undefined keys
+  // are dropped (unchanged) and null writes NULL. An empty string from a
+  // cleared form field must therefore become null, or clearing does nothing.
+  it('maps empty-string expense account to null so clearing persists', () => {
+    const result = UpdateSupplierSchema.safeParse({ default_expense_account: '' })
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.default_expense_account).toBeNull()
+    }
+  })
+
+  it('maps empty-string email to null so clearing persists', () => {
+    const result = UpdateSupplierSchema.safeParse({ email: '' })
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.email).toBeNull()
+    }
+  })
+
+  it('leaves omitted fields absent', () => {
+    const result = UpdateSupplierSchema.safeParse({ name: 'New Supplier' })
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect('default_expense_account' in result.data).toBe(false)
+      expect('email' in result.data).toBe(false)
+    }
+  })
+
+  it('still rejects a malformed expense account on update', () => {
+    const result = UpdateSupplierSchema.safeParse({ default_expense_account: '54' })
+    expect(result.success).toBe(false)
+  })
+
   it('rejects invalid expense account format', () => {
     const result = UpdateSupplierSchema.safeParse({ default_expense_account: '40' })
     expect(result.success).toBe(false)