[Feature Request]: Providing Flatpak installer for Linux desktop

### What would you like to happen?

Hi, Team,

Flatpak is a sandbox desktop environments that can ship software without too much dependency burdon.

It may be great to package into Flatpak sandbox and distribute in Flathub and provide more way to attract users.

And there are some samples that package Java-based software into it.
https://github.com/flathub/io.github.yairm210.unciv/
https://github.com/flathub/com.shatteredpixel.shatteredpixeldungeon/

### Issue Priority

Priority: 3

### Issue Component

Component: Hop Gui

Start by reviewing the Hop Gui packaging and distribution setup, then compare it with the linked Flatpak examples for Java applications. Done means a Flatpak package for Hop Gui is available and can be distributed through Flathub.

Written by the indexing model from the issue text.