fix: Support both MEDUSA_BACKEND_URL and NEXT_PUBLIC_MEDUSA_BACKEND_URL in middleware

[Vallabh-Mahanta]

Description

This PR fixes issue #484 where the middleware only accepts MEDUSA_BACKEND_URL environment variable but not NEXT_PUBLIC_MEDUSA_BACKEND_URL, which causes confusion and errors when setting up the storefront.

Changes

• Updated the middleware to check for both MEDUSA_BACKEND_URL and NEXT_PUBLIC_MEDUSA_BACKEND_URL environment variables
• Updated error messages to be more clear about the environment variables needed
• Updated the .env.template file to include both variables with explanations
• Updated README.md to document both environment variables

Testing

Tested by running the application with only NEXT_PUBLIC_MEDUSA_BACKEND_URL defined, which now works correctly.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Documentation update (fixing inaccurate or missing information)

Affected Components

• Middleware
• Documentation

Related Issues

Fixes #484

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