Initial install of heroic on steam deck no full screen in game mode

[coro-bot]

          Issue occurs on steam deck still. Version 2.15.2

Poor initial user experience, required me to search and look through github "closed/completed bugs" from a year ago to find fix. Normal users may drop heroic launcher because of this bad initial setup experience.

Reproduction steps:
install flatpak
open in desktop mode (heroic launcher remembers non maximized version)
close
change to game mode
open heroic launcher
Screen not maximized and mouse input cannot reach edges, there is also not an obvious "fullscreen" button

Required user action:
add " --fullscreen" to end of launch options.
this is unintuitive when heroic launcher already recognizes that I'm on a steam deck and what SKU I have.

Originally posted by @coro-bot in #2850 (comment)