Add Premium Plan and Sign-in Feature

[naipaka]

Overview

Implement premium plan subscription system and user sign-in functionality.

Requirements

Premium Plan Page

• Create a premium plan page

Sign-in Functionality

• Implement user sign-in functionality

Premium Plan Subscription

• Implement premium plan subscription functionality
• Users must be signed in to subscribe
• Link subscription to the signed-in account
• Integrate with RevenueCat
• Subscription status should persist across devices and sign-in/sign-out sessions