Adjust Geolocate control UI

* On mobile, the button is too close to the nav links.
* Nav bar can sometimes cover fixed-position elements, possibly correct with something like [this](https://github.com/adobe/react-spectrum/blob/main/packages/%40react-aria/utils/src/useViewportSize.ts)
* Could hide the icon when auto follow is active. This is a common pattern for Google/Apple Maps.
* Geolocate button does not appear on iPad. Thanks @jas_williams

Start at the Geolocate control and nav layout, then compare the viewport-positioning approach in the linked useViewportSize reference. Check the mobile spacing, nav overlap, auto-follow icon behavior, and iPad visibility; the work is done when each reported UI problem is addressed across the affected layouts.

Written by the indexing model from the issue text.