Allow configuring mobile controller UI skip duration (fixes #26704)

Currently the seek buttons in the mobile web controller UI seek by a fixed 30 seconds. This is one of the buttons I'm referring to:

This merge request changes the buttons so that they jump by the "Short jump length" value configured in Interface > Hotkeys settings.

Fixes #26704 (closed)

Some notes

• I did this by exposing the short jump length value to the JavaScript in mobile.html. A different way I could have done this would be to add two new commands called e.g. seek_backwards_short and seek_forward_short which would look up the configured duration and seek by it. I think the way I did it is more flexible for future changes, but happy to do it the other way if the maintainers disagree.
• I chose to seek by the short jump length instead of e.g. the medium jump length because the short jump length is the "default" jump length that you get when you press the arrow keys without any modifiers. Note that since the default short jump length is 10 seconds, this will change the out-of-the-box behavior of the mobile controller UI.
• I wasn't sure if there were any backwards-compatibility considerations related to adding more fields to status.xml. I figured I'd open the MR and let y'all tell me if I need to fix anything.