fix(screenshots): refresh app-icon-1024.png with the new master

The previous screenshots/app-icon-1024.png on this branch was a copy
of the OLD upstream MX Control Suite raster (77132 bytes, 256x256
upscaled to 1024 in display), not the new icon. The "Screenshots"
section in PR TomBadash#177 therefore embedded the icon the PR claims to be
replacing, which is the opposite of useful.

Replace it with the actual new master from
images/logo_icon.png at HEAD of feat/app-icon-refresh: 1024x1024
RGBA, navy gradient squircle on a 100 px transparent gutter with
the Phosphor mouse-simple glyph in white.

No other screenshot on this branch needed updating -- the sidebar
shot (app-icon-refresh-sidebar.png) already reflects the new
sidebar brand mark.

Author: hughesyadaddy