feat: add pin lock / fix performance on history view

lib/services/pin_lock_service.dart

@@ -0,0 +1,56 @@
+import 'dart:convert';
+import 'dart:math';
+
+import 'package:crypto/crypto.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+/// Stores and verifies a PIN. The raw PIN is never persisted — only a
+/// salted SHA-256 hash of it, kept in secure storage (Keychain on iOS,
+/// EncryptedSharedPreferences/Keystore on Android).
+class PinLockService {
+  PinLockService({FlutterSecureStorage? storage})
+      : _storage = storage ?? const FlutterSecureStorage();
+
+  static const _saltKey = 'pin_salt';
+  static const _hashKey = 'pin_hash';
+
+  final FlutterSecureStorage _storage;
+
+  Future<bool> hasPin() async {
+    final hash = await _storage.read(key: _hashKey);
+    return hash != null && hash.isNotEmpty;
+  }
+
+  Future<void> setPin(String pin) async {
+    final salt = _generateSalt();
+    final hash = _hash(pin, salt);
+
+    await _storage.write(key: _saltKey, value: salt);
+    await _storage.write(key: _hashKey, value: hash);
+  }
+
+  Future<bool> verifyPin(String pin) async {
+    final salt = await _storage.read(key: _saltKey);
+    final storedHash = await _storage.read(key: _hashKey);
+
+    if (salt == null || storedHash == null) return false;
+
+    return _hash(pin, salt) == storedHash;
+  }
+
+  Future<void> clearPin() async {
+    await _storage.delete(key: _saltKey);
+    await _storage.delete(key: _hashKey);
+  }
+
+  String _generateSalt([int length = 16]) {
+    final random = Random.secure();
+    final bytes = List<int>.generate(length, (_) => random.nextInt(256));
+    return base64UrlEncode(bytes);
+  }
+
+  String _hash(String pin, String salt) {
+    final bytes = utf8.encode('$salt:$pin');
+    return sha256.convert(bytes).toString();
+  }
+}