Add Linux Flatpak build to CI workflow

[sanderland]

Summary

• Add Flatpak packaging files (manifest, desktop entry, metainfo XML, icon) and a build-flatpak CI job that produces a .flatpak bundle artifact using the official flatpak/flatpak-github-actions action
• Pull in dependabot bumps: urllib3 2.5.0 → 2.6.3, pillow 11.2.1 → 11.3.0

Test plan

• CI build-flatpak job completes successfully
• Flatpak bundle artifact is uploaded and downloadable
• flatpak install --user KaTrain-*.flatpak && flatpak run org.katrain.KaTrain launches the app
• KataGo engine starts inside the sandbox
• SGF files can be loaded/saved

🤖 Generated with Claude Code

[tquid]

This should fix the build, however, the resulting flatpak silently segfaults on me on my local. I think it might be an X11/Wayland issue, but I'm still working on that bit. In any case if you can get the build passing, at least people with more flatpak experience might be able to help with debugging:

--- a/flatpak/org.katrain.KaTrain.yaml
+++ b/flatpak/org.katrain.KaTrain.yaml
@@ -21,8 +21,8 @@ modules:
       - --disable-static
     sources:
       - type: archive
-        url: https://gitlab.freedesktop.org/libevdev/mtdev/-/archive/v1.1.7/mtdev-v1.1.7.tar.gz
-        sha256: 50f6ed402789219e75ccfe72ab5aa52d21512e376406105566cc693fcd1114dc
+        url: https://bitmath.org/code/mtdev/mtdev-1.1.7.tar.bz2
+        sha256: a107adad2101fecac54ac7f9f0e0a0dd155d954193da55c2340c97f2ff1d814e
 
   - name: libzip
     buildsystem: cmake-ninja

[sanderland]

@tquid 🚀

[tquid]

There is a more fundamental issue here. Since AppImages use FUSE filesystem access, they fail when you try to execute them inside a flatpak, because the flatpak sandbox doesn't allow that. The cure is to unspool the appimage with --appimage-extract, but we can't only do that with the build manifest. Since the user has options to grab katago appimages at runtime, there need to be hooks in there as well, that basically go "am I in a flatpak", and if so, do the appimage extract and set appropriate paths.

I don't know enough about flatpaks and appimage architecture, nor about KaTrain's code, to implement something like that. But it seems like the next obstacle here.

Edit: here's what that error looks like:

ERROR: Unexpected exception Expecting value: line 1 column 1 (char 0) while processing KataGo output b'Error: No suitable fusermount binary found on the $PATH'
This doesn't look like a squashfs image.
open dir error: No such file or directory
ERROR: Engine died unexpectedly without sending output:  status 127

[tquid]

The appimage thing isn't too tough to fix, seems like, but I eventually hit a wall trying to get Kivy to deal with Wayland. Even with XWayland running to do pretend X11, I can't get the flatpak to even bring up a GUI.