Build the Flatpak in CI and publish it to the Frostyard remote

[bketelsen]

Summary

Adds .github/workflows/flatpak.yml:

• Every push and pull request: builds io.github.frostyard.Igloo.json in ghcr.io/flathub-infra/flatpak-github-actions:gnome-51, runs the meson tests inside the build (run-tests: true), and uploads the bundle as the igloo-flatpak artifact. Dependencies such as VTE are cached by the manifest's hash.
• Pushes to main: also export the build with flatpak build-bundle --oci and push it to ghcr.io/frostyard/igloo as latest and sha-COMMIT.

frostyard/flatpak-index serves those images as an OCI remote from GitHub Pages, the approach tuna-os uses. It reads the images anonymously, so this repository needs no secrets. The optional FLATPAK_INDEX_DISPATCH_TOKEN secret (a fine-grained token with Actions write on frostyard/flatpak-index) refreshes the remote right after publishing; without it, the remote's hourly run picks up new images.

The README gains an Install section for the remote and for a pull request's bundle.

After merging

• The first push to main creates the igloo package. If GitHub creates it as private, make it public once in its package settings; the remote skips private images.

• Then install with:

  flatpak remote-add --if-not-exists frostyard https://frostyard.github.io/flatpak-index/frostyard.flatpakrepo
  flatpak install frostyard io.github.frostyard.Igloo

Testing

• actionlint passes.
• Rehearsed the build job locally with the same manifest change the action makes (run-tests on the last module): all five meson tests pass inside the build, and flatpak build-bundle --oci produces an image whose labels carry the ref, metadata, AppStream data and 64/128 px icons.
• The index builder in frostyard/flatpak-index produced, from GHCR, the same entry tuna-os's index has for tuna-os/mariner, and flatpak remote-ls listed it from a local copy of the generated site.
• This PR's own run is the first real CI build.

🤖 Generated with Claude Code