feat(vat): show already-booked banner when opening momsdeklaration (#1703)

The settlement check only ran on step 3, so Granska recalculated boxes with no signal that a vat_settlement (or momsomforing) already existed. Load the proposal with the report and reuse that detection for a top banner plus the stepper.

Signed-off-by: Daniel Stenborg <daniel@stenborg.se>
Co-authored-by: Cursor <cursoragent@cursor.com>

lib/reports/__tests__/vat-settlement.test.ts

@@ -1,5 +1,12 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest'
-import { buildVatSettlementProposal } from '../vat-settlement'
+import {
+  buildVatSettlementProposal,
+  findDraftVatSettlement,
+  findPostedVatSettlement,
+  vatDeadlineTaxPeriod,
+  vatSettlementBookingStatus,
+  type VatSettlementExistingEntry,
+} from '../vat-settlement'
 
 // ============================================================
 // Mock: fetchVatAccountTotals now goes through the
@@ -304,3 +311,46 @@ describe('buildVatSettlementProposal', () => {
     ).rejects.toThrow('existing vat_settlement lookup failed: boom')
   })
 })
+
+describe('vat settlement UI gate helpers', () => {
+  const posted: VatSettlementExistingEntry = {
+    id: 'je-posted',
+    status: 'posted',
+    entry_date: '2026-06-30',
+    source_type: 'vat_settlement',
+    voucher_series: 'A',
+    voucher_number: 83,
+  }
+  const draft: VatSettlementExistingEntry = {
+    id: 'je-draft',
+    status: 'draft',
+    entry_date: '2026-06-30',
+    source_type: 'vat_settlement',
+    voucher_series: 'A',
+    voucher_number: 84,
+  }
+
+  it('picks the posted settlement over a draft', () => {
+    expect(findPostedVatSettlement([draft, posted])).toEqual(posted)
+    expect(findDraftVatSettlement([draft, posted])).toBeUndefined()
+    expect(vatSettlementBookingStatus([draft, posted])).toBe('booked')
+  })
+
+  it('surfaces a draft only when nothing is posted', () => {
+    expect(findPostedVatSettlement([draft])).toBeUndefined()
+    expect(findDraftVatSettlement([draft])).toEqual(draft)
+    expect(vatSettlementBookingStatus([draft])).toBe('draft')
+  })
+
+  it('is none when the period has no settlement', () => {
+    expect(findPostedVatSettlement([])).toBeUndefined()
+    expect(vatSettlementBookingStatus([])).toBe('none')
+    expect(vatSettlementBookingStatus(undefined)).toBe('none')
+  })
+
+  it('formats the calendar tax_period for monthly and quarterly VAT', () => {
+    expect(vatDeadlineTaxPeriod('monthly', 2026, 6)).toBe('2026-06')
+    expect(vatDeadlineTaxPeriod('quarterly', 2026, 2)).toBe('2026-Q2')
+    expect(vatDeadlineTaxPeriod('yearly', 2026, 1)).toBeNull()
+  })
+})