Implement local vault service with encryption key management and integrate it into the app. Add settings screen for data management and enhance home screen with new features. Update database connection for encryption support and modify repository to use the new database structure. Improve UI elements across the application for better user experience.

lib/data/local_vault_service.dart

@@ -0,0 +1,56 @@
+import 'dart:math';
+
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+class LocalVaultService {
+  LocalVaultService._();
+
+  static final LocalVaultService instance = LocalVaultService._();
+
+  static const String _encryptionKeyStorageKey =
+      'notes_local_encryption_key_v1';
+
+  final FlutterSecureStorage _secureStorage = FlutterSecureStorage();
+
+  String? _cachedEncryptionKey;
+
+  Future<String?> readEncryptionKey() async {
+    final String? cachedKey = _cachedEncryptionKey;
+    if (cachedKey != null) {
+      return cachedKey;
+    }
+
+    final String? storedKey = await _secureStorage.read(
+      key: _encryptionKeyStorageKey,
+    );
+
+    _cachedEncryptionKey = storedKey;
+    return storedKey;
+  }
+
+  Future<String> createEncryptionKey() async {
+    final String encryptionKey = _generateEncryptionKey();
+
+    await _secureStorage.write(
+      key: _encryptionKeyStorageKey,
+      value: encryptionKey,
+    );
+
+    _cachedEncryptionKey = encryptionKey;
+    return encryptionKey;
+  }
+
+  Future<void> clearEncryptionKey() async {
+    await _secureStorage.delete(key: _encryptionKeyStorageKey);
+    _cachedEncryptionKey = null;
+  }
+
+  String _generateEncryptionKey() {
+    final Random random = Random.secure();
+    final List<int> bytes = List<int>.generate(32, (_) => random.nextInt(256));
+
+    return bytes
+        .map((int byte) => byte.toRadixString(16).padLeft(2, '0'))
+        .join();
+  }
+}