Remove SyncService and introduce context-based offline mode and state management

- Delete `SyncService` and its associated bidirectional synchronization logic.
- Add multiple context providers (`OfflineProvider`, `AlertProvider`, `LangContext`, `UserContext`, `SessionContext`, `WorldContext`, `SettingBookContext`) for contextual state management.
- Implement `SecureStorage` for OS-level secure data encryption and replace dependency on `SyncService` synchronization.
- Update localization files (`en.json`, `fr.json`) with offline mode and error-related strings.

context/LangContext.ts

@@ -0,0 +1,14 @@
+import {Context, createContext, Dispatch, SetStateAction} from "react";
+
+export type SupportedLocale = 'fr' | 'en';
+
+export interface LangContextProps {
+    lang: SupportedLocale;
+    setLang: Dispatch<SetStateAction<SupportedLocale>>;
+}
+
+export const LangContext: Context<LangContextProps> = createContext<LangContextProps>({
+    lang: 'fr',
+    setLang: (): void => {
+    }
+});