Remove DataService and OfflineDataService, refactor book and character operations to use streamlined handlers in LocalSystem

- Delete `data.service.ts` and `offline-data.service.ts`, consolidating functionality into `LocalSystem`.
- Refactor book, character, and conversation operations to adopt unified, multilingual, and session-enabled IPC handlers in `LocalSystem`.
- Simplify redundant legacy methods, enhancing maintainability and consistency.

electron/main.ts

@@ -6,6 +6,9 @@ import Store from 'electron-store';
 import * as fs from 'fs';
 import { getDatabaseService } from './database/database.service.js';
 
+// Import IPC handlers
+import './ipc/book.ipc.js';
+
 // Fix pour __dirname en ES modules
 const __filename = fileURLToPath(import.meta.url);
 const __dirname = path.dirname(__filename);