Floating Action Button

## Summary
Overlaid button for highlighting key actions regardless of scroll position.

### Edits by @GCHQ-Developer-718 08/08/2025

There have been at least two internal requests for a sticky button that scrolls a component into view when clicked. A specific request I've received was for a sticky icon button that scrolls to a chat window on click and works with the ICDS footer and classification banner.

As the back to top component already provides this functionality, it would be helpful to make a customisable version of this to suit the customer's use-case, which has all the variants of ic-button.

An "ic-sticky-button" could have a target prop like ic-back-to-top for the above use-case. If target is used, an anchor element should be rendered.

It could also accept a click event handler for the use-case mentioned in the original summary and, in this case, a button element should be rendered.

Start by reading the existing ic-back-to-top component and the ic-button variants. Define the customizable sticky button around the stated target and click-handler behaviors, including anchor versus button rendering and compatibility with the ICDS footer and classification banner. Done means both interaction modes and the requested variants are supported.

Written by the indexing model from the issue text.