feat: implement label entity validators

Author: ethicnology

lib/features/labels/domain/label_entity.dart

@@ -1,4 +1,6 @@
 import 'package:bb_mobile/features/labels/primitive/label_type.dart';
+import 'package:bs58check/bs58check.dart' as base58;
+import 'package:convert/convert.dart';
 
 class LabelEntity {
   final int? id;
@@ -16,18 +18,80 @@ class LabelEntity {
     this.origin,
     this.spendable,
   }) {
+    _validateReference();
+  }
+
+  void _validateReference() {
     switch (type) {
       case LabelType.transaction:
-        if (reference.length != 64) throw Exception('Invalid tx reference');
+        _validateTxid(reference);
       case LabelType.address:
-      // deserialize using bdk and lwk
+        // Address validation requires async BDK/LWK calls
+        break;
       case LabelType.publicKey:
       case LabelType.input:
-      // check txid:int
+        final parts = reference.split(':');
+        _validateTxid(parts[0]);
+        _validateIndex(parts[1]);
+
       case LabelType.output:
-      // check txid:int
+        final parts = reference.split(':');
+        _validateTxid(parts[0]);
+        _validateIndex(parts[1]);
       case LabelType.extendedPublicKey:
-      // base58 decode size == 78
+        _validateExtendedPublicKeyReference();
+    }
+  }
+
+  void _validateTxid(String input) {
+    if (reference.length != 64) {
+      throw LabelValidationException(
+        'Invalid transaction reference: must be 64 hex characters',
+      );
+    }
+
+    try {
+      hex.decode(reference);
+    } catch (e) {
+      throw LabelValidationException(
+        'Invalid transaction reference: must be valid hex',
+      );
+    }
+  }
+
+  void _validateIndex(String input) {
+    final index = int.tryParse(input);
+    if (index == null || index < 0) {
+      throw LabelValidationException(
+        'Invalid index reference: must be a non-negative integer',
+      );
+    }
+  }
+
+  /// Validates extended public key reference by decoding base58 and checking length
+  /// Extended keys (xpub, ypub, zpub, tpub, etc.) are 78 bytes when decoded
+  void _validateExtendedPublicKeyReference() {
+    try {
+      final decoded = base58.decode(reference);
+      if (decoded.length != 78) {
+        throw LabelValidationException(
+          'Invalid extended public key reference: decoded length must be 78 bytes, got ${decoded.length}',
+        );
+      }
+    } catch (e) {
+      if (e is LabelValidationException) rethrow;
+      throw LabelValidationException(
+        'Invalid extended public key reference: failed to decode base58 - $e',
+      );
     }
   }
 }
+
+class LabelValidationException implements Exception {
+  final String message;
+
+  LabelValidationException(this.message);
+
+  @override
+  String toString() => 'LabelValidationException: $message';
+}