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/data/local_vault_service.dart

@@ -74,6 +74,14 @@ class LocalVaultService {
     return _secureStorage.read(key: _encryptionKeyStorageKey);
   }
 
+  Future<void> storeEncryptionKey(String encryptionKey) async {
+    await _secureStorage.write(key: _encryptionKeyStorageKey, value: encryptionKey);
+  }
+
+  String generateEncryptionKey() {
+    return _generateEncryptionKey();
+  }
+
   Future<bool> hasEncryptionKey() async {
     return (await readStoredEncryptionKeyRaw()) != null;
   }
@@ -117,10 +125,7 @@ class LocalVaultService {
   Future<String> createEncryptionKey({bool protectWithBiometrics = false}) async {
     final String encryptionKey = _generateEncryptionKey();
 
-    await _secureStorage.write(
-      key: _encryptionKeyStorageKey,
-      value: encryptionKey,
-    );
+    await storeEncryptionKey(encryptionKey);
 
     // If requested, try to enable biometric protection. Only enable on mobile
     // platforms and only if the authentication succeeds.