feat: add foreign currency support, refactor bookkeeping engine, and improve invoice inbox document classification

- Add currency-utils module for SEK conversion with exchange rates
- Refactor createJournalEntry to use draft+commit flow preventing voucher number gaps (BFL 5 kap. 7§)
- Add foreign currency support to invoice entries with per-line SEK conversion
- Centralize category-to-account mapping into single source of truth
- Refactor invoice inbox to use shared document analyzer with document type classification (receipt, supplier invoice, government letter)
- Update mapping engine, supplier invoice entries, and transaction entries
- Fix report component rendering issues
- Add new validation schemas and tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/api/invoices/[id]/mark-paid/__tests__/route.test.ts

@@ -126,7 +126,8 @@ describe('POST /api/invoices/[id]/mark-paid', () => {
       expect.anything(),
       'user-1',
       expect.objectContaining({ id: 'inv-1' }),
-      expect.any(String)
+      expect.any(String),
+      undefined
     )
   })
 

app/api/invoices/[id]/mark-paid/route.ts

@@ -4,6 +4,7 @@ import {
   createInvoicePaymentJournalEntry,
   createInvoiceCashEntry,
 } from '@/lib/bookkeeping/invoice-entries'
+import { MarkInvoicePaidSchema } from '@/lib/api/schemas'
 import type { EntityType, Invoice } from '@/types'
 
 /**
@@ -49,8 +50,24 @@ export async function POST(
     )
   }
 
+  // Parse optional body (backward compatible — body may be empty)
+  let exchangeRateDifference: number | undefined
+  let bodyPaymentDate: string | undefined
+  try {
+    const text = await request.text()
+    if (text) {
+      const parsed = MarkInvoicePaidSchema.safeParse(JSON.parse(text))
+      if (parsed.success) {
+        exchangeRateDifference = parsed.data.exchange_rate_difference
+        bodyPaymentDate = parsed.data.payment_date
+      }
+    }
+  } catch {
+    // No body or invalid JSON — use defaults
+  }
+
   const now = new Date().toISOString()
-  const paymentDate = now.split('T')[0]
+  const paymentDate = bodyPaymentDate || now.split('T')[0]
 
   // Update status to paid
   const { error: updateError } = await supabase
@@ -89,7 +106,8 @@ export async function POST(
           supabase,
           user.id,
           invoice as Invoice,
-          paymentDate
+          paymentDate,
+          exchangeRateDifference
         )
         journalEntryId = journalEntry?.id ?? null
       } else {