`instance-create.tsx`: button inconsistency

I was using the instance-create.tsx form to create an instance when I noticed the buttons for disks, SSH keys, and floating IPs are all inconsistent with one another. We should consider making all these buttons consistent so it's clear they can be clicked to perform the desired action. I would suggest making them all use the primary variant, but perhaps that would conflict with the "Create instance" button that's on the bottom of the entire form. Either way, I think some consistency would be good here.

Reference

The disks section uses two buttons, the variants are primary and secondary.

The SSH keys section uses a single button with the ghost variant.

The floating IPs section uses a single button with the secondary variant.

This is just close enough visually to a disabled button, which confused me for a bit because I thought I couldn't click the button even though I had valid floating IPs available.

Start in instance-create.tsx and compare the buttons in the disks, SSH keys, and floating IPs sections, including their primary, secondary, and ghost variants. Choose a consistent treatment that remains distinct from the form's Create instance button and does not resemble a disabled control. Done means the three sections use a clear, consistent button style.