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/database/models/Book.ts

@@ -254,7 +254,7 @@ export default class Book {
         return BookRepo.insertBook(id,userId,encryptedTitle,hashedTitle,encryptedSubTitle,hashedSubTitle,encryptedSummary,type,serie,publicationDate,desiredWordCount,lang);
     }
     
-    public static async getBook(userId:string,bookId: string, lang: 'fr' | 'en' = 'fr'): Promise<BookProps> {
+    public static async getBook(userId:string,bookId: string): Promise<BookProps> {
         const book:Book = new Book(bookId);
         await book.getBookInfos(userId);
         return {