test(invoice): add money, invoice and rendering tests

test/invoice_render_test.dart

@@ -0,0 +1,52 @@
+import 'package:cellar/cellar.dart';
+import 'package:test/test.dart';
+
+bool isAscii(String text) => text.runes.every((rune) => rune < 128);
+
+Invoice invoice() => Invoice(
+      number: 'FV/2026/07/001',
+      issueDate: DateTime(2026, 8, 28),
+      dueDate: DateTime(2026, 9, 11),
+      seller: const Party(name: 'Vulp Sp. z o.o.', address: 'Warszawa', taxId: 'PL1234567890'),
+      buyer: const Party(name: 'Acme <Corp>', address: 'Krakow'),
+      currency: Currency.pln,
+      paymentTerms: '14 days',
+      lines: [
+        InvoiceLine(
+          description: 'Consulting',
+          quantity: 12,
+          unitPrice: Money.of(150, Currency.pln),
+          vat: VatRate.standard,
+        ),
+        InvoiceLine(
+          description: 'Manual',
+          quantity: 3,
+          unitPrice: Money.of(20, Currency.pln),
+          vat: VatRate.reduced5,
+        ),
+      ],
+    );
+
+void main() {
+  group('invoice text render', () {
+    test('shows the totals and stays ASCII', () {
+      final text = renderInvoiceText(invoice());
+      expect(text.contains('INVOICE FV/2026/07/001'), isTrue);
+      expect(text.contains('Subtotal (net): 1 860.00 zl'), isTrue);
+      expect(text.contains('VAT total:      417.00 zl'), isTrue);
+      expect(text.contains('TOTAL:          2 277.00 zl'), isTrue);
+      expect(isAscii(text), isTrue);
+    });
+  });
+
+  group('invoice HTML render', () {
+    test('is a full ASCII document with the total', () {
+      final html = renderInvoiceHtml(invoice());
+      expect(html.contains('<!DOCTYPE html>'), isTrue);
+      expect(html.contains('</html>'), isTrue);
+      expect(html.contains('2 277.00 zl'), isTrue);
+      expect(html.contains('Acme &lt;Corp&gt;'), isTrue);
+      expect(isAscii(html), isTrue);
+    });
+  });
+}