Add a ubuntu-24.04 linux-flatpak job that runs build_flatpak.sh from the
shared love payload, wire it into release staging/upload, and mirror a
path-gated Flatpak build in ci.yml when flatpak packaging changes.
Drop the pointless zip wrapper around the x86_64 AppImage, harden HostShell
so bundled curl keeps APPDIR libs while host curl scrubs LD_LIBRARY_PATH /
Steam LD_PRELOAD, and add a Flatpak channel with --device=all, bundled
curl, and AppStream releases metadata. Portable mode now probes writability
with a unique temp file and falls back soft on RO mounts / Flatpak.