feat: semi-manual invoice payment booking dialog (#37)

* fix: include reversed entries in all reports (general ledger, trial balance, VAT, SIE, NE, INK2)

Reversed entries (storno) must appear alongside their original posted entries
in reports for a complete audit trail. Previously, filtering by status='posted'
excluded them, causing discrepancies when corrections had been made.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: semi-manual invoice payment booking with editable journal lines

When marking an invoice as paid, users now see a dialog where they can:
- Choose which bank/cash account the payment goes to (1910, 1920, 1930, etc.)
- Review and edit the proposed journal entry lines before committing
- The happy path remains fast — lines are pre-filled correctly

Implementation:
- Pure proposePaymentLines() function for line computation (accrual + cash)
- PaymentBookingDialog with AccountCombobox, balance validation, date picker
- API accepts optional custom lines, falls back to auto-generation without them
- 18 tests (8 unit + 10 API) all passing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: address Greptile review — validation fallback, balance check, error handling

- P1: Return 400 on invalid body instead of silently falling back to
  auto-generated lines (split JSON parse from schema validation)
- P1: Add server-side balance check for custom lines before committing
  (debit must equal credit, totalDebit > 0)
- P2: Wrap PaymentBookingDialog init() in try/catch with toast on
  failure and auto-close instead of silent empty state
- Add 2 new tests: unbalanced lines → 400, invalid schema → 400

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/api/schemas.ts

@@ -174,6 +174,12 @@ export const MarkInvoicePaidSchema = z.object({
   payment_date: isoDate.optional(),
   exchange_rate_difference: z.number().optional(),
   notes: z.string().optional(),
+  lines: z.array(z.object({
+    account_number: accountNumber,
+    debit_amount: nonNegativeAmount.default(0),
+    credit_amount: nonNegativeAmount.default(0),
+    line_description: z.string().optional(),
+  })).min(2).optional(),
 })
 
 // ============================================================