Add Stripe Portal button to homepage

[homanp]

This pull request implements the Stripe Portal functionality by adding a button to the homepage that allows users to access their Stripe billing portal.

Changes made:

1. Added a "Open Stripe Portal" button to the homepage.
2. Implemented a handleStripePortal function to handle the button click event.
3. The function makes a POST request to /api/stripe-portal to create a portal session.
4. On successful response, it redirects the user to the Stripe portal URL.
5. Added error handling for failed requests.

Note: This PR only includes the frontend changes. A separate PR will be needed to implement the backend API route /api/stripe-portal to create the Stripe portal session.

TODO:

• Implement the /api/stripe-portal API route to create a Stripe portal session.
• Ensure that the Stripe customer ID is dynamically fetched for the logged-in user.
• Add proper authentication to ensure only authenticated users can access the Stripe portal.

Testing:

• Verify that the "Open Stripe Portal" button appears on the homepage.
• Click the button and ensure it triggers the handleStripePortal function.
• Once the backend is implemented, test the full flow to make sure users are redirected to their Stripe portal.