feat: agregar clase PasswordHasher para el hash de contraseñas

lib/core/security/password_hasher.dart

@@ -0,0 +1,13 @@
+import 'dart:convert';
+
+import 'package:crypto/crypto.dart';
+
+class PasswordHasher {
+  static const String _salt = 'waste-notify-local-salt-v1';
+
+  static String hash(String password) {
+    final normalized = password.trim();
+    final bytes = utf8.encode('$_salt:$normalized');
+    return sha256.convert(bytes).toString();
+  }
+}