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>

lib/api/__tests__/schemas.test.ts

@@ -23,6 +23,7 @@ import {
   CreateInvoiceItemSchema,
   CreateInvoiceSchema,
   CreateCreditNoteSchema,
+  MarkInvoicePaidSchema,
   // Customer schemas
   CreateCustomerSchema,
   // Supplier schemas
@@ -423,6 +424,47 @@ describe('CreateCreditNoteSchema', () => {
   })
 })
 
+describe('MarkInvoicePaidSchema', () => {
+  it('accepts empty object (all fields optional)', () => {
+    const result = MarkInvoicePaidSchema.safeParse({})
+    expect(result.success).toBe(true)
+  })
+
+  it('accepts payment_date', () => {
+    const result = MarkInvoicePaidSchema.safeParse({ payment_date: '2024-07-15' })
+    expect(result.success).toBe(true)
+  })
+
+  it('accepts exchange_rate_difference (positive gain)', () => {
+    const result = MarkInvoicePaidSchema.safeParse({ exchange_rate_difference: 200 })
+    expect(result.success).toBe(true)
+  })
+
+  it('accepts exchange_rate_difference (negative loss)', () => {
+    const result = MarkInvoicePaidSchema.safeParse({ exchange_rate_difference: -300 })
+    expect(result.success).toBe(true)
+  })
+
+  it('accepts all fields together', () => {
+    const result = MarkInvoicePaidSchema.safeParse({
+      payment_date: '2024-07-15',
+      exchange_rate_difference: 150.50,
+      notes: 'Paid via Wise',
+    })
+    expect(result.success).toBe(true)
+  })
+
+  it('rejects invalid payment_date format', () => {
+    const result = MarkInvoicePaidSchema.safeParse({ payment_date: '15/07/2024' })
+    expect(result.success).toBe(false)
+  })
+
+  it('rejects non-number exchange_rate_difference', () => {
+    const result = MarkInvoicePaidSchema.safeParse({ exchange_rate_difference: 'big gain' })
+    expect(result.success).toBe(false)
+  })
+})
+
 // ============================================================
 // Customer schemas
 // ============================================================