Migrate from electron-store to OS-level secure storage (getSecureStorage)

- Replace `electron-store` with OS-level encrypted storage for secure token, userId, and language management in `LocalSystem` and `keyManager`.
- Add `init-user` IPC handler to initialize user data and manage encryption keys.
- Update login process to handle encrypted storage saving with fallback for macOS issues.
- Add offline warning component to `login/page.tsx` to handle first-time sync requirements.
- Remove `electron-store` and associated dependencies from `package.json` and `package-lock.json`.

electron.d.ts

@@ -23,9 +23,12 @@ export interface IElectronAPI {
   setLang: (lang: 'fr' | 'en') => Promise<void>;
 
   // Auth events (one-way communication)
-  loginSuccess: (token: string, userId: string) => void;
+  loginSuccess: (token: string) => void;
   logout: () => void;
 
+  // User initialization (after getting user info from server)
+  initUser: (userId: string) => Promise<{ success: boolean; keyCreated?: boolean; error?: string }>;
+
   // Encryption key management (shortcuts for convenience)
   generateEncryptionKey: (userId: string) => Promise<string>;
   getUserEncryptionKey: (userId: string) => Promise<string | null>;