Update the project's dependency from isomorphic-fetch to cross-fetch

[nozomione]

Context

To prevent the Module not found: Can't resolve 'unfetch' error, replace isomorphic-fetch (not actively maintained) with cross-fetch.

We'll address this issue before the Next.js app router migration begins.

Problem or idea

Update the project dependency listed above.

Solution or next step

• Add cross-fetch
• Remove isomorphic-fetch
• Adjust the import statements