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.d.ts

@@ -1,27 +1,38 @@
+/**
+ * TypeScript declarations for window.electron API
+ * Must match exactly with electron/preload.ts
+ *
+ * Usage:
+ * - Use invoke<T>(channel, ...args) for all IPC calls
+ * - Shortcuts are provided for common operations (tokens, lang, encryption)
+ */
 export interface IElectronAPI {
+  // Platform info
   platform: NodeJS.Platform;
+
+  // Generic invoke method - use this for all IPC calls
+  invoke: <T = any>(channel: string, ...args: any[]) => Promise<T>;
+
+  // Token management (shortcuts for convenience)
   getToken: () => Promise<string | null>;
-  setToken: (token: string) => Promise<boolean>;
-  removeToken: () => Promise<boolean>;
-  loginSuccess: (token: string) => void;
+  setToken: (token: string) => Promise<void>;
+  removeToken: () => Promise<void>;
+
+  // Language management (shortcuts for convenience)
+  getLang: () => Promise<'fr' | 'en'>;
+  setLang: (lang: 'fr' | 'en') => Promise<void>;
+
+  // Auth events (one-way communication)
+  loginSuccess: (token: string, userId: string) => void;
   logout: () => void;
 
-  // Database operations
-  generateEncryptionKey: (userId: string) => Promise<{ success: boolean; key?: string; error?: string }>;
+  // Encryption key management (shortcuts for convenience)
+  generateEncryptionKey: (userId: string) => Promise<string>;
   getUserEncryptionKey: (userId: string) => Promise<string | null>;
-  setUserEncryptionKey: (userId: string, encryptionKey: string) => Promise<boolean>;
-  dbInitialize: (userId: string, encryptionKey: string) => Promise<{ success: boolean; error?: string }>;
-  dbGetBooks: () => Promise<{ success: boolean; data?: any[]; error?: string }>;
-  dbGetBook: (bookId: string) => Promise<{ success: boolean; data?: any; error?: string }>;
-  dbSaveBook: (book: any, authorId?: string) => Promise<{ success: boolean; error?: string }>;
-  dbDeleteBook: (bookId: string) => Promise<{ success: boolean; error?: string }>;
-  dbSaveChapter: (chapter: any, bookId: string, contentId?: string) => Promise<{ success: boolean; error?: string }>;
-  dbGetCharacters: (bookId: string) => Promise<{ success: boolean; data?: any[]; error?: string }>;
-  dbSaveCharacter: (character: any, bookId: string) => Promise<{ success: boolean; error?: string }>;
-  dbGetConversations: (bookId: string) => Promise<{ success: boolean; data?: any[]; error?: string }>;
-  dbSaveConversation: (conversation: any, bookId: string) => Promise<{ success: boolean; error?: string }>;
-  dbGetSyncStatus: () => Promise<{ success: boolean; data?: any[]; error?: string }>;
-  dbGetPendingChanges: (limit?: number) => Promise<{ success: boolean; data?: any[]; error?: string }>;
+  setUserEncryptionKey: (userId: string, encryptionKey: string) => Promise<void>;
+
+  // Database initialization (shortcut for convenience)
+  dbInitialize: (userId: string, encryptionKey: string) => Promise<boolean>;
 }
 
 declare global {