Add local backup/restore: encrypted .enc export to Download/loginvault, restore from list (no file_picker to avoid SDK conflict)

lib/encryptor.dart

@@ -26,4 +26,20 @@ class Encryptor {
     final xored = _xorBytes(base64.decode(cipher), keyBytes);
     return utf8.decode(xored);
   }
+
+  /// Encrypt [plain] with a user password (for backup files).
+  /// Uses the password bytes as the XOR key so the backup is protected by the
+  /// app password, not the in-app constant key.
+  static String encryptWithPassword(String plain, String password) {
+    final keyBytes = utf8.encode(password);
+    final xored = _xorBytes(utf8.encode(plain), keyBytes);
+    return base64.encode(xored);
+  }
+
+  /// Decrypt a backup [cipher] produced by [encryptWithPassword].
+  static String decryptWithPassword(String cipher, String password) {
+    final keyBytes = utf8.encode(password);
+    final xored = _xorBytes(base64.decode(cipher), keyBytes);
+    return utf8.decode(xored);
+  }
 }