fix: implement persistent auth session using expo-secure-store

[Midoriya-w]

Closes #264

Changes:

• Implemented persistent auth session using expo-secure-store
• Token is saved to secure storage on login
• Token is restored automatically on app start via useEffect
• User profile is re-fetched if saved token exists
• Token is cleared from secure storage on logout
• If saved token is invalid/expired, storage is cleared automatically
• Replaces all TODO placeholders with working implementation

[Harxhit]

@ShantKhatri Could please look into this.