feat(invoice): add invoice json loader

lib/src/invoice/loader.dart

@@ -0,0 +1,105 @@
+import 'dart:convert';
+
+import 'invoice.dart';
+import 'money.dart';
+
+Invoice invoiceFromJsonString(String source) =>
+    invoiceFromJson(jsonDecode(source) as Map<String, dynamic>);
+
+Invoice invoiceFromJson(Map<String, dynamic> json) {
+  final currency = Currency.byCode(json['currency'] as String? ?? 'PLN');
+  final linesJson = (json['lines'] as List).cast<Map<String, dynamic>>();
+  final lines = [
+    for (final line in linesJson)
+      InvoiceLine(
+        description: line['description'] as String,
+        quantity: line['quantity'] as num,
+        unitPrice: Money.of((line['unitPrice'] as num).toDouble(), currency),
+        vat: _vatFrom(line['vat']),
+        discountRate: (line['discount'] as num?)?.toDouble() ?? 0.0,
+        unit: line['unit'] as String? ?? 'pcs',
+      ),
+  ];
+  return Invoice(
+    number: json['number'] as String,
+    issueDate: _parseDate(json['issueDate'] as String),
+    dueDate: json['dueDate'] == null ? null : _parseDate(json['dueDate'] as String),
+    seller: _partyFrom(json['seller'] as Map<String, dynamic>),
+    buyer: _partyFrom(json['buyer'] as Map<String, dynamic>),
+    currency: currency,
+    lines: lines,
+    notes: json['notes'] as String?,
+    paymentTerms: json['paymentTerms'] as String?,
+    reportingCurrency:
+        json['reportingCurrency'] == null ? null : Currency.byCode(json['reportingCurrency'] as String),
+    reportingRate: (json['reportingRate'] as num?)?.toDouble(),
+  );
+}
+
+String encodeInvoiceJson(Invoice invoice) {
+  final json = <String, dynamic>{
+    'number': invoice.number,
+    'issueDate': _formatDate(invoice.issueDate),
+    if (invoice.dueDate != null) 'dueDate': _formatDate(invoice.dueDate!),
+    'currency': invoice.currency.code,
+    'seller': _partyToJson(invoice.seller),
+    'buyer': _partyToJson(invoice.buyer),
+    if (invoice.paymentTerms != null) 'paymentTerms': invoice.paymentTerms,
+    if (invoice.notes != null) 'notes': invoice.notes,
+    if (invoice.reportingCurrency != null) 'reportingCurrency': invoice.reportingCurrency!.code,
+    if (invoice.reportingRate != null) 'reportingRate': invoice.reportingRate,
+    'lines': [
+      for (final line in invoice.lines)
+        {
+          'description': line.description,
+          'quantity': line.quantity,
+          'unitPrice': line.unitPrice.amount,
+          'vat': line.vat.rate == 0 ? line.vat.label : (line.vat.rate * 100),
+          if (line.discountRate != 0) 'discount': line.discountRate,
+          'unit': line.unit,
+        },
+    ],
+  };
+  return const JsonEncoder.withIndent('  ').convert(json);
+}
+
+Party _partyFrom(Map<String, dynamic> json) => Party(
+      name: json['name'] as String,
+      address: json['address'] as String?,
+      taxId: json['taxId'] as String?,
+      email: json['email'] as String?,
+    );
+
+Map<String, dynamic> _partyToJson(Party party) => {
+      'name': party.name,
+      if (party.address != null) 'address': party.address,
+      if (party.taxId != null) 'taxId': party.taxId,
+      if (party.email != null) 'email': party.email,
+    };
+
+VatRate _vatFrom(Object? value) {
+  if (value is num) return VatRate.fromPercent(value);
+  final label = (value as String).trim().toLowerCase();
+  switch (label) {
+    case 'zw':
+    case 'exempt':
+      return VatRate.exempt;
+    case '0':
+    case '0%':
+      return VatRate.zero;
+    default:
+      final numeric = double.tryParse(label.replaceAll('%', ''));
+      if (numeric != null) return VatRate.fromPercent(numeric);
+      throw FormatException('Unknown VAT rate: $value');
+  }
+}
+
+DateTime _parseDate(String text) {
+  final parts = text.split('-');
+  return DateTime(int.parse(parts[0]), int.parse(parts[1]), int.parse(parts[2]));
+}
+
+String _formatDate(DateTime date) =>
+    '${date.year.toString().padLeft(4, '0')}-'
+    '${date.month.toString().padLeft(2, '0')}-'
+    '${date.day.toString().padLeft(2, '0')}';