feat(invoices): per-line percentage discount and separate fakturamarkning (#2084)

* feat(invoices): per-line percentage discount and separate fakturamarkning

User request: rabatt i procent per artikelrad, and a marking field
separate from Er referens.

- invoice_items.discount_percent (0-100, default 0): line_total and
  vat_amount are stored NET of the discount. Shared exact-ore math in
  lib/invoices/line-amounts.ts (gross, discount, net) used by the web
  builder, staged-operation commit, editor preview, PDF, and Peppol.
  Undiscounted lines keep the legacy unrounded qty*price byte-identical.
- ROT/RUT deduction computes on the discounted net line total.
- invoices.invoice_marking: printed on the PDF next to the references
  and mapped to Peppol BT-10 BuyerReference (marking wins over
  your_reference; either satisfies the BT-10 requirement).
- Peppol renders the discount as a BG-27 line AllowanceCharge
  (reason code 95, MultiplierFactorNumeric, Amount, BaseAmount).
- Editor: "Lagg till rabatt" in the row menu (same reveal pattern as
  ROT/RUT), Markning row next to Er referens, forval chip, review
  dialog shows discounts and marking.
- Plumbed through v1 REST projections, MCP create/get/update invoice
  tools, pending-operations update path, and copy-invoice (discount
  copied; marking deliberately not, it is recipient-specific).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJAt9yM7tgZ69f1XnNnq52

* fix(invoices): carry discount_percent through every deduction, credit, convert and preview path

Skeptic + CI findings on the discount/marking feature, one pass:

- generateRotRutLines and propose-send-lines now pass discount_percent
  into computeDeduction: the send/credit/cash verifikat booked 1513 on
  the GROSS line while deduction_total, the PDF and the Skatteverket
  claim carried the net, stranding the difference on 1513 and pushing
  1510 negative once the customer paid. Test pins 1513=3000/1510=7000
  for a 20%-discounted 10 000 kr ROT line.
- preview-pdf route accepts discount_percent (net totals + net-based
  deduction) and invoice_marking; the editor now sends the marking, so
  the preview equals the invoice it becomes.
- Credit notes carry discount_percent (buildCreditNoteItem, v1 credit
  route select+insert, MCP credit executor) and invoice_marking, so the
  kreditfaktura face arithmetic multiplies out and shows the Rabatt
  column (ML 17 kap 24 §).
- Proforma->invoice convert copies discount_percent + invoice_marking:
  the converted invoice previously failed Peppol LINE_TOTAL_MISMATCH
  and lost the rebate on the next builder pass.
- Editor hides the discount menu in self-billed mode (the self-billed
  wire shape has no discount; previewed net would book gross).
- MCP staging and commitCreateInvoice reject a non-number
  discount_percent (a string coerced past the range check but was
  ignored by the totals math and still stored).
- Regenerated skills/accounted-api (apiskill:check CI failure).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJAt9yM7tgZ69f1XnNnq52

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

components/invoices/__tests__/invoice-editor-flow.test.ts

@@ -165,6 +165,7 @@ function chipsInput(overrides: Partial<ForvalChipsInput> = {}): ForvalChipsInput
     receivedDate: '',
     deliveryDate: '',
     yourReference: '',
+    invoiceMarking: '',
     paymentLink: null,
     oreRounding: true,
     dims: null,
@@ -221,6 +222,7 @@ describe('deriveForvalChips', () => {
         currency: 'EUR',
         deliveryDate: '2026-08-20',
         yourReference: 'Anna',
+        invoiceMarking: 'KST 4711',
         paymentLink: 'manual',
         oreRounding: false,
         dims: 'KS01 · P001',
@@ -230,6 +232,7 @@ describe('deriveForvalChips', () => {
     expect(chips).toContainEqual({ kind: 'currency', currency: 'EUR' })
     expect(chips).toContainEqual({ kind: 'delivery', date: '2026-08-20' })
     expect(chips).toContainEqual({ kind: 'your_reference', reference: 'Anna' })
+    expect(chips).toContainEqual({ kind: 'invoice_marking', marking: 'KST 4711' })
     expect(chips).toContainEqual({ kind: 'payment_link', mode: 'manual' })
     expect(chips).toContainEqual({ kind: 'dims', dims: 'KS01 · P001' })
     // ore_off is SEK-only: an EUR invoice has no öresavrundning to disable.
@@ -249,6 +252,7 @@ describe('deriveForvalChips', () => {
         receivedDate: '2026-08-15',
         documentType: 'proforma',
         yourReference: 'x',
+        invoiceMarking: 'x',
         paymentLink: 'auto',
         oreRounding: false,
         dims: 'KS01',