feat: Add Stellar address validation utility

lib/screens/wallet_screen.dart

@@ -3,6 +3,7 @@ import 'package:flutter/services.dart';
 import 'package:provider/provider.dart';
 
 import '../providers/wallet_provider.dart';
+import '../utils/validation.dart';
 import '../widgets/transaction_button.dart';
 
 class WalletScreen extends StatefulWidget {
@@ -24,9 +25,14 @@ class _WalletScreenState extends State<WalletScreen> {
 
   void _onConnect(WalletProvider provider) {
     final key = _secretKeyController.text.trim();
-    if (key.isEmpty) return;
+    final error = Validation.stellarSecretKey(key);
+    if (error != null) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text(error), backgroundColor: Colors.red),
+      );
+      return;
+    }
     provider.connect(key);
-    // Clear the field immediately so the key is not held in memory longer than needed
     _secretKeyController.clear();
   }
 

lib/utils/validation.dart

@@ -0,0 +1,26 @@
+class Validation {
+  /// Returns null if valid, or an error message if invalid.
+  static String? stellarSecretKey(String key) {
+    final trimmed = key.trim();
+    if (trimmed.isEmpty) return 'Secret key is required';
+    if (!trimmed.startsWith('S')) return "Secret key must start with 'S'";
+    if (trimmed.length != 56) {
+      return 'Secret key must be exactly 56 characters (got ${trimmed.length})';
+    }
+    final base32 = RegExp(r'^[A-Z2-7]+$');
+    if (!base32.hasMatch(trimmed)) {
+      return 'Secret key contains invalid characters (must be A-Z and 2-7)';
+    }
+    return null;
+  }
+
+  static String? stellarPublicKey(String key) {
+    final trimmed = key.trim();
+    if (trimmed.isEmpty) return 'Address is required';
+    if (!trimmed.startsWith('G')) return "Stellar address must start with 'G'";
+    if (trimmed.length != 56) {
+      return 'Stellar address must be exactly 56 characters (got ${trimmed.length})';
+    }
+    return null;
+  }
+}

test/utils/validation_test.dart

@@ -0,0 +1,51 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:nodus_protocol/utils/validation.dart';
+
+void main() {
+  group('Validation.stellarSecretKey', () {
+    test('valid 56-char S-key returns null', () {
+      const key = 'SCZANGBA5AKIA7ORYH4RO2EQVN3ISYIKXT6EU7EOSQ37NQBP5M4OBK3A';
+      expect(Validation.stellarSecretKey(key), isNull);
+    });
+
+    test('empty string fails', () {
+      expect(Validation.stellarSecretKey(''), isNotNull);
+    });
+
+    test('wrong prefix fails', () {
+      expect(Validation.stellarSecretKey('GAAAAAAABBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB'), isNotNull);
+    });
+
+    test('wrong length fails', () {
+      expect(Validation.stellarSecretKey('SSHORT'), isNotNull);
+    });
+
+    test('invalid base32 chars fail', () {
+      expect(Validation.stellarSecretKey('S${'1' * 55}'), isNotNull);
+    });
+
+    test('valid key with spaces trims and validates', () {
+      const key = ' SCZANGBA5AKIA7ORYH4RO2EQVN3ISYIKXT6EU7EOSQ37NQBP5M4OBK3A ';
+      expect(Validation.stellarSecretKey(key), isNull);
+    });
+  });
+
+  group('Validation.stellarPublicKey', () {
+    test('valid 56-char G-key returns null', () {
+      const key = 'GAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAXJH';
+      expect(Validation.stellarPublicKey(key), isNull);
+    });
+
+    test('empty string fails', () {
+      expect(Validation.stellarPublicKey(''), isNotNull);
+    });
+
+    test('wrong prefix fails', () {
+      expect(Validation.stellarPublicKey('SAAAAAAABBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB'), isNotNull);
+    });
+
+    test('wrong length fails', () {
+      expect(Validation.stellarPublicKey('GSHORT'), isNotNull);
+    });
+  });
+}