"Waiting for Mint" Enhancement

[TravisLeeRichardson]

Issue:
Code was hanging on "waiting for mint" (line 121 of App.js).
The nft object was null when calling the mint function.

The fix:
We need to ensure that the nft object is initialized properly before the user can submit the form.

The changes:

1. Disable the "Create & Mint" button by default.
2. Add a new state variable to track if the minting is enabled.
3. Update the useEffect to watch for changes in the nft object and enable the button accordingly.