Add BooksSyncContext, refine database schema, and enhance synchronization support

- Introduce `BooksSyncContext` for managing book synchronization states (server-only, local-only, to-sync, etc.).
- Remove `UserContext` and related dependencies.
- Refine localization strings (`en.json`) with sync-related updates (e.g., "toSyncFromServer", "toSyncToServer").
- Extend database schema with additional tables and fields for syncing books, chapters, and related entities.
- Add `last_update` fields and update corresponding repository methods to support synchronization logic.
- Enhance IPC handlers with stricter typing, data validation, and sync-aware operations.

electron/database/System.ts

@@ -16,6 +16,11 @@ export default class System {
         return encryptDataWithUserKey(data, userKey);
     }
     
+    public static timeStampInSeconds(): number {
+        const date:number = new Date().getTime();
+        return Math.floor(date / 1000);
+    }
+    
     public static decryptDataWithUserKey(encryptedData: string, userKey: string): string {
         return decryptDataWithUserKey(encryptedData, userKey);
     }