`Upload` button on mobile should display warning/info

Since https://frameright.app works only on desktop, it is unlikely that, as of today, the user will have pictures with Image Regions on their phone. Thus when clicking on `Upload` on their phone, we should tell them to better switch to desktop for now.

Start by locating the Upload control and the code that handles its click on mobile devices. Show an informational warning directing mobile users to switch to desktop, while preserving the existing upload behavior on desktop; confirm the warning appears when Upload is clicked on a phone.

Written by the indexing model from the issue text.