feat(invoices): registrera utan att bokföra + explicit Bokför-steg (#1040)

* feat(invoices): registrera utan att bokföra + explicit Bokför-steg

Companies where one person registers supplier invoices / sends customer
invoices while ekonomi does the actual bookkeeping had no way to split
the two: under faktureringsmetoden every registration/send booked the
journal entry inline.

- New company setting defer_invoice_booking (default off, accrual only):
  registering a supplier invoice or sending/marking-sent a customer
  invoice creates NO journal entry.
- New explicit booking routes POST /api/supplier-invoices/[id]/book and
  POST /api/invoices/[id]/book: create the registration/revenue entry
  afterwards, CAS-guarded against concurrent booking (a lost race
  cancels the just-posted voucher with a gap explanation), including
  periodisering schedules.
- Detail pages show "Ej bokförd ännu" + a Bokför button for unbooked
  accrual invoices; the settings toggle lives under Bokföringsmetod.
- mark-paid needs no changes: both payment flows already route on the
  journal-entry link, so an invoice still unbooked when paid gets the
  full cash-style entry.
- The mark-sent fail-closed rollback now keys on the same gate so
  deferred sends are not rolled back as booking failures.

Fixes #967

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

* fix(invoices): harden deferred booking after review

CodeRabbit round on #1040:
- CAS link guards also require a still-bookable status (and uncredited,
  customer side) so a concurrent mark-paid/credit cannot end up with a
  double-posting registration/revenue entry.
- Settings reads fail closed instead of defaulting to accrual rules.
- Detail pages surface the ACCRUAL_SCHEDULE_FAILED warning instead of
  showing plain success, and the customer page no longer stringifies
  structured errors into "[object Object]".
- The settings form normalizes defer_invoice_booking to false under
  kontantmetoden so a stale flag cannot re-activate on method switch.

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

---------

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

lib/api/schemas.ts

@@ -1466,6 +1466,8 @@ export const UpdateSettingsSchema = z.object({
   iban: z.string().regex(/^SE\d{22}$/, 'Ogiltigt IBAN (SE följt av 22 siffror)').nullable().optional().or(z.literal('')),
   bic: z.string().regex(/^[A-Z]{6}[A-Z0-9]{2}([A-Z0-9]{3})?$/, 'Ogiltig BIC/SWIFT (8 eller 11 tecken)').nullable().optional().or(z.literal('')),
   accounting_method: AccountingMethodSchema.optional(),
+  // #967: register/send invoices without booking; booking is a separate step.
+  defer_invoice_booking: z.boolean().optional(),
   invoice_prefix: z.string().nullable().optional(),
   next_invoice_number: z.number().int().positive().optional(),
   next_arrival_number: z.number().int().positive().optional(),