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/AlertProvider.tsx

@@ -0,0 +1,80 @@
+'use client';
+
+import type {Context, Dispatch, JSX, ReactNode, SetStateAction} from 'react';
+import {createContext, useCallback, useState} from 'react';
+import AlertStack from '@/components/AlertStack';
+
+export type AlertType = 'success' | 'error' | 'info' | 'warning';
+
+export interface Alert {
+    id: string;
+    type: AlertType;
+    message: string;
+}
+
+export interface AlertContextProps {
+    successMessage: (message: string) => void;
+    errorMessage: (message: string) => void;
+    infoMessage: (message: string) => void;
+    warningMessage: (message: string) => void;
+}
+
+interface AlertProviderProps {
+    children: ReactNode;
+}
+
+export const AlertContext: Context<AlertContextProps> = createContext<AlertContextProps>({
+    successMessage: (_message: string): void => {
+    },
+    errorMessage: (_message: string): void => {
+    },
+    infoMessage: (_message: string): void => {
+    },
+    warningMessage: (_message: string): void => {
+    },
+});
+
+export function AlertProvider({children}: AlertProviderProps): JSX.Element {
+    const [alerts, setAlerts]: [Alert[], Dispatch<SetStateAction<Alert[]>>] = useState<Alert[]>([]);
+    
+    const addAlert: (type: AlertType, message: string) => void = useCallback((type: AlertType, message: string): void => {
+        const id: string = `${Date.now()}-${Math.random().toString(36).substring(2, 11)}`;
+        const newAlert: Alert = {id, type, message};
+        
+        setAlerts((prev: Alert[]): Alert[] => [...prev, newAlert]);
+    }, []);
+    
+    const removeAlert: (id: string) => void = useCallback((id: string): void => {
+        setAlerts((prev: Alert[]): Alert[] => prev.filter((alert: Alert): boolean => alert.id !== id));
+    }, []);
+    
+    const successMessage: (message: string) => void = useCallback((message: string): void => {
+        addAlert('success', message);
+    }, [addAlert]);
+    
+    const errorMessage: (message: string) => void = useCallback((message: string): void => {
+        addAlert('error', message);
+    }, [addAlert]);
+    
+    const infoMessage: (message: string) => void = useCallback((message: string): void => {
+        addAlert('info', message);
+    }, [addAlert]);
+    
+    const warningMessage: (message: string) => void = useCallback((message: string): void => {
+        addAlert('warning', message);
+    }, [addAlert]);
+    
+    return (
+        <AlertContext.Provider
+            value={{
+                successMessage,
+                errorMessage,
+                infoMessage,
+                warningMessage,
+            }}
+        >
+            {children}
+            <AlertStack alerts={alerts} onClose={removeAlert}/>
+        </AlertContext.Provider>
+    );
+}