Linux desktop support via AppImage

[bradleystachurski]

Adds Linux desktop distribution via AppImage. Users get a single portable ~86MB file that runs across Linux distros without installation. When we tag releases, CI automatically builds and attaches the AppImage to release assets alongside the APK (I'll verify this works during v0.5.0-rc testing rather than testing here).

Testing locally:

# Build portable AppImage
just build-appimage

# Run the AppImage
# - Non-NixOS: just open the .AppImage file directly
# - NixOS:
just run-appimage-nixos ./ecash-app-*.AppImage

Notable changes:

• Linux data directory moved to ~/.local/share/ecash-app/ following XDG Base Directory conventions. Debug builds use ecash-app-dev/ for isolation so dev testing doesn't clobber release data. Android behavior unchanged.

• Docker build for portability. Binaries built from nix develop embed /nix/store/ interpreter paths that only run on systems with those exact Nix store paths. The Docker-based just build-appimage produces binaries with standard Linux paths that work everywhere. CI uses flutter-action on ubuntu-latest for the same reason.

• AppStream metadata included. This is standard Linux software center metadata (GNOME Software, KDE Discover, etc). After v0.5.0 ships with desktop support, we can submit to AppImageHub for broader discovery. They auto-scrape new releases after initial submission so there's almost no additional release admin overhead compared to what we do now. No app store submissions, just a release asset in the tag.

• Release process updated to include AppStream version entries for final releases.

[m1sterc001guy]

Where is the icon used? Is it for the submission to one of the app directories? At least for me, after building it looked like this:

[m1sterc001guy]

Huge. Tested on my machine and it works!

[m1sterc001guy]

Is this missing?

[bradleystachurski]

Ended up removing this and using only the docker build so there's only one AppImage build path and it's portable

[bradleystachurski]

Where is the icon used? Is it for the submission to one of the app directories? At least for me, after building it looked like this:

What does that screenshot point to? Is that the Flutter bundle output? The icon gets embedded in the final .AppImage file after appimagetool runs. Let me know if you don't see an icon with just build-appimage or just build-appimage-portable.

[bradleystachurski]

Tested on fresh Ubuntu 24.04 VM with GNOME and reproduced. The icon is embedded correctly (verified via --appimage-extract). GNOME doesn't display embedded icons for AppImage files in the file manager. Kdenlive's official AppImage has the same generic gear icon. KDE Plasma displays it correctly.

I think it's okay to move forward since this is the same behavior for other popular AppImages on GNOME, but let me know if you think it's worth continuing to find a workaround.