chore: integrate Hive for local storage management

- Added `hive_ce` and `hive_ce_flutter` dependencies for enhanced local storage capabilities.
- Refactored the main application to initialize Hive and migrate existing data.
- Updated storage service implementations to utilize Hive for managing application settings and task queues.
- Removed the deprecated `StorageService` class to streamline the codebase and improve maintainability.

lib/core/persistence/hive_boxes.dart

@@ -0,0 +1,35 @@
+import 'package:hive_ce/hive.dart';
+
+/// Logical names for Hive boxes used by the app.
+final class HiveBoxNames {
+  static const String preferences = 'preferences_v1';
+  static const String caches = 'caches_v1';
+  static const String attachmentQueue = 'attachment_queue_v1';
+  static const String metadata = 'metadata_v1';
+}
+
+/// Well-known keys stored inside Hive boxes.
+final class HiveStoreKeys {
+  // Metadata
+  static const String migrationVersion = 'migration_version';
+
+  // Cache entries
+  static const String localConversations = 'local_conversations';
+  static const String attachmentQueueEntries = 'attachment_queue_entries';
+  static const String taskQueue = 'outbound_task_queue_v1';
+}
+
+/// Grouped Hive boxes that remain open for the app lifecycle.
+class HiveBoxes {
+  HiveBoxes({
+    required this.preferences,
+    required this.caches,
+    required this.attachmentQueue,
+    required this.metadata,
+  });
+
+  final Box<dynamic> preferences;
+  final Box<dynamic> caches;
+  final Box<dynamic> attachmentQueue;
+  final Box<dynamic> metadata;
+}