Publish to Flathub

[arferreira]

Flatpak build works locally on Arch. Submit to Flathub for proper distribution.

[arferreira]

@ShiraiEd need you to generate the cargo-sources.json for the Flathub submission. Sandboxed builds have no network so we need vendored deps.

From the repo root:

pip install flatpak-cargo-generator
flatpak-cargo-generator linux/Cargo.lock -o cargo-sources.json

Push it to a branch and open a PR.