feat(cryptographic): add jwt API method

lib/src/cryptographic.dart

@@ -110,4 +110,35 @@ class Cryptographic {
     );
     return words.join(' ');
   }
+
+  /// Returns JWT-like token structure.
+  ///
+  /// [payload] is optional JWT payload (claims). If none provided default
+  /// payload is used.
+  ///
+  /// [algorithm] is optional JWT algorithm (default is "HS256").
+  ///
+  /// Example:
+  /// ```dart
+  /// Cryptographic().jwt(); // "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiI3NjZmZGFjNi0yNjU3LTQ1NTQtYTNiMy01MGI2ODIyOTRmNmUiLCJuYW1lIjoiVGVzdCBVc2VyIiwiaWF0IjoxNzY3OTY5NDM2LCJleHAiOjE3Njc5NzMwMzZ9.OLkALspgIX982pMKceErTpSOwNLSv-Uhtyu-ZO7yi5U"
+  /// ```
+  String jwt({Map<String, dynamic>? payload, String algorithm = 'HS256'}) {
+    final header = {'alg': algorithm, 'typ': 'JWT'};
+
+    final now = DateTime.now();
+    final iat = now.millisecondsSinceEpoch ~/ 1000;
+    final exp = iat + 3600;
+
+    payload ??= {'sub': uuid, 'name': 'Test User', 'iat': iat, 'exp': exp};
+
+    final headerJson = jsonEncode(header);
+    final payloadJson = jsonEncode(payload);
+
+    final base64UrlEncoder = utf8.fuse(base64Url);
+    final header64 = base64UrlEncoder.encode(headerJson).replaceAll('=', '');
+    final payload64 = base64UrlEncoder.encode(payloadJson).replaceAll('=', '');
+    final signature = base64UrlEncode(tokenBytes()).replaceAll('=', '');
+
+    return '$header64.$payload64.$signature';
+  }
 }

test/src/cryptographic_test.dart

@@ -1,3 +1,5 @@
+import 'dart:convert';
+
 import 'package:imitatio/imitatio.dart';
 import 'package:test/test.dart';
 
@@ -84,5 +86,75 @@ void main() {
       expect(result.split(' ').length, inInclusiveRange(12, 24));
       expect(seededCrypto.mnemonicPhrase, equals(seededCrypto.mnemonicPhrase));
     });
+
+    test('returns JWT with default params', () {
+      final result = crypto.jwt();
+      final parts = result.split('.');
+      expect(parts.length, equals(3));
+
+      final headerJson = base64.decode(base64Url.normalize(parts[0]));
+      final header =
+          jsonDecode(utf8.decode(headerJson)) as Map<String, dynamic>;
+      expect(header['alg'], equals('HS256'));
+      expect(header['typ'], equals('JWT'));
+
+      final payloadJson = base64.decode(base64Url.normalize(parts[1]));
+      final payload =
+          jsonDecode(utf8.decode(payloadJson)) as Map<String, dynamic>;
+      expect(payload['sub'], isNotEmpty);
+      expect(payload['name'], equals('Test User'));
+      expect(payload['iat'], isA<int>());
+      expect(payload['exp'], isA<int>());
+
+      expect(seededCrypto.jwt(), equals(seededCrypto.jwt()));
+    });
+
+    test('returns JWT with provided algorithm', () {
+      const algorithm = 'HS512';
+
+      final result = crypto.jwt(algorithm: algorithm);
+      final parts = result.split('.');
+      expect(parts.length, equals(3));
+
+      final headerJson = base64.decode(base64Url.normalize(parts[0]));
+      final header =
+          jsonDecode(utf8.decode(headerJson)) as Map<String, dynamic>;
+      expect(header['alg'], equals(algorithm));
+
+      expect(
+        seededCrypto.jwt(algorithm: algorithm),
+        equals(seededCrypto.jwt(algorithm: algorithm)),
+      );
+    });
+
+    test('returns JWT with provided payload', () {
+      const payloads = [
+        {'user_id': 123, 'role': 'admin'},
+        {
+          'sub': '[email protected]',
+          'permissions': ['read', 'write'],
+        },
+        {
+          'custom_field': 'value',
+          'nested': {'key': 'value'},
+        },
+        {'empty': null, 'bool': true, "number": 42},
+      ];
+
+      for (final p in payloads) {
+        final result = crypto.jwt(payload: p);
+        final parts = result.split('.');
+        expect(parts.length, equals(3));
+
+        final payloadJson = base64.decode(base64Url.normalize(parts[1]));
+        final payload =
+            jsonDecode(utf8.decode(payloadJson)) as Map<String, dynamic>;
+        expect(payload, equals(p));
+        expect(
+          seededCrypto.jwt(payload: p),
+          equals(seededCrypto.jwt(payload: p)),
+        );
+      }
+    });
   });
 }