fix(booking): explicit Ingen moms in quick review books exempt, not the derived default (#1450)

QuickReviewDialog collapsed 'none' to undefined on the wire, so the server
re-derived the category default and booked 25% moms against an explicit
no-VAT choice while the preview showed no VAT line. 'none' now maps to
'exempt' only when it deviates from the seeded default: untouched bookings
on exempt-default categories (bank fees) stay byte-identical, an explicit
Ingen moms books no VAT line with the classification the momsdeklaration
should see, and the class-2 account auto-'none' stops booking a stray moms
line. Same gap the swedish-compliance bot flagged on #1443.

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

lib/transactions/__tests__/quick-review-defaults.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest'
-import { resolveQuickReviewDefaults, type ReviewTemplate } from '../quick-review-defaults'
+import { resolveQuickReviewDefaults, resolveExplicitVat, type ReviewTemplate } from '../quick-review-defaults'
 import { getDefaultAccountForCategory } from '@/lib/bookkeeping/category-mapping'
 
 /**
@@ -70,3 +70,24 @@ describe('resolveQuickReviewDefaults', () => {
     expect(vat === 'none' || typeof vat === 'string').toBe(true)
   })
 })
+
+describe('resolveExplicitVat', () => {
+  it('passes explicit rate-bearing treatments through unchanged', () => {
+    expect(resolveExplicitVat('standard_25', 'standard_25')).toBe('standard_25')
+    expect(resolveExplicitVat('reduced_12', 'standard_25')).toBe('reduced_12')
+    expect(resolveExplicitVat('reverse_charge', 'none')).toBe('reverse_charge')
+  })
+
+  it("keeps a seeded 'none' default off the wire (server derives, no VAT line)", () => {
+    // Bank fees etc.: category default is null -> displayed as 'none'.
+    // Untouched submit must stay byte-identical to the pre-fix behavior.
+    expect(resolveExplicitVat('none', 'none')).toBeUndefined()
+  })
+
+  it("sends explicit 'exempt' when the user deviates to Ingen moms", () => {
+    // The old collapse to undefined made the server re-derive the category
+    // default and book 25% moms against an explicit no-VAT choice.
+    expect(resolveExplicitVat('none', 'standard_25')).toBe('exempt')
+    expect(resolveExplicitVat('none', 'reduced_12')).toBe('exempt')
+  })
+})

lib/transactions/quick-review-defaults.ts

@@ -70,3 +70,31 @@ export function resolveQuickReviewDefaults(
 
   return { account, vat }
 }
+
+/**
+ * Resolve the VAT treatment the review dialog should put on the wire.
+ *
+ * 'none' is a UI-only sentinel with two meanings that must not be conflated:
+ *
+ *  - As the SEEDED default (exempt categories like bank fees, or a template
+ *    without a treatment): send nothing and let the server derive, exactly as
+ *    before. The derivation yields no VAT line for those categories, and
+ *    keeping the wire empty preserves byte-identical bookings for the common
+ *    untouched case.
+ *  - As a DEVIATION (the user explicitly picked "Ingen moms" on a category
+ *    whose default carries VAT, or the dialog auto-set 'none' for a class-2
+ *    account override): send 'exempt'. The old collapse to undefined made the
+ *    server re-derive the default, silently booking 25% moms against an
+ *    explicit no-VAT choice, while the preview showed no VAT line. An explicit
+ *    'exempt' books no VAT line and records the classification the
+ *    momsdeklaration should see (see buildMappingResultFromCategory's note);
+ *    for income it also lands revenue on 3004 instead of a rate-bearing
+ *    account.
+ */
+export function resolveExplicitVat(
+  selected: VatTreatment | 'none',
+  seededDefault: VatTreatment | 'none',
+): VatTreatment | undefined {
+  if (selected !== 'none') return selected
+  return seededDefault === 'none' ? undefined : 'exempt'
+}