"Start app on login" fails

Hi thanks a lot for this packaging, very helpful :)

I'm using it on Linux Mint, here the "Start app on login" setting does not work: it writes an autostart script with the command `/app/main/mattermost-desktop --hidden` which of course makes no sense from the pov of my system.

I ended up disabling the setting and adding my own autostart entry which calls the same command as the existing menu entry: `/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=mattermost-flatpak --file-forwarding com.mattermost.Desktop --enable-features=WebRTCPipeWireCapturer`

Would be nice if this autostart command could be adjusted accordingly when added by a flatpak'd app.

Inspect how the "Start app on login" setting generates its autostart entry, then compare it with the existing menu command: `/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=mattermost-flatpak --file-forwarding com.mattermost.Desktop --enable-features=WebRTCPipeWireCapturer`. Done means enabling the setting creates a usable Linux startup entry instead of invoking `/app/main/mattermost-desktop --hidden`.

Written by the indexing model from the issue text.