feat: Implement note encryption and synchronization features

- Added NoteEncryption class for encrypting and decrypting note content using AES-GCM.
- Updated NoteRepository to handle synchronization of notes and categories with the server, including encryption of note data before sending.
- Introduced SyncRequest and SyncResponse models for managing synchronization data.
- Enhanced LocalVaultService to store and retrieve the encryption key.
- Modified HomeScreen and SettingsScreen to trigger synchronization after note operations and manage API endpoint settings.
- Added SyncStatusIndicator to provide visual feedback on synchronization status in the app title bar.
- Created Category model to manage note categories with encryption support.
- Updated note model to include UUID, server version, deletion status, and category ID.
- Added necessary UI elements for displaying and managing the encryption key in SettingsScreen.
- Updated dependencies in pubspec.yaml for cryptography and HTTP handling.

lib/models/category.dart

@@ -0,0 +1,45 @@
+import 'package:uuid/uuid.dart';
+
+class Category {
+  Category({
+    String? uuid,
+    required this.encryptedName,
+    this.serverVersion = 0,
+    this.isDeleted = false,
+    required this.updatedAt,
+  }) : uuid = uuid ?? Uuid().v4();
+
+  final String uuid;
+  final String encryptedName;
+  final int serverVersion;
+  final bool isDeleted;
+  final DateTime updatedAt;
+
+  Category copyWith({
+    String? uuid,
+    String? encryptedName,
+    int? serverVersion,
+    bool? isDeleted,
+    DateTime? updatedAt,
+  }) {
+    return Category(
+      uuid: uuid ?? this.uuid,
+      encryptedName: encryptedName ?? this.encryptedName,
+      serverVersion: serverVersion ?? this.serverVersion,
+      isDeleted: isDeleted ?? this.isDeleted,
+      updatedAt: updatedAt ?? this.updatedAt,
+    );
+  }
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) {
+      return true;
+    }
+
+    return other is Category && uuid == other.uuid;
+  }
+
+  @override
+  int get hashCode => uuid.hashCode;
+}