Add Next/Prev buttons to gradient controls

[thaedal]

Summary

This PR adds "Next" and "Prev" buttons to the gradient control section, allowing users to quickly cycle through the available gradient options without repeatedly opening the dropdown menu.

Changes

• Modified index.html to include two new buttons: Prev and Next, located after the "Invert" checkbox.
• Added JavaScript event listeners for these buttons to increment or decrement the selectedIndex of the gradient dropdown.
• Added boundary checks to ensure the selection stops at the beginning or end of the list without errors.
• Triggering these buttons automatically updates the gradient preview and the colourised output.

[Coffee-Nerd]

Hey bud, did you know you can already do this by using the up/down arrows with your keyboard?

I've actually got many HUGE updates to the colorizer coming soon. (Been pending for like 6 months, got busy with my client)

AND a NEW colorizer as well!

Thanks for the commit, I'll have a look at it later today.