feat(snap): Snap Store packaging

[Nozzit]

Summary

• Adds snap/snapcraft.yaml + .github/workflows/snap.yml, same pattern as open-planner-studio: re-packages the already-published release .deb instead of rebuilding, triggered on completion of Build and Release.
• Publishing to the Snap Store only happens once SNAPCRAFT_STORE_CREDENTIALS is set as a repo secret.
• Snap name: openaec-installer.

⚠️ Two things to check before this is actually useful

1. Name not registered yet. Nobody has registered openaec-installer on snapcraft.io — needs to happen before the publish step can do anything.
2. Confinement risk, worth flagging explicitly. This app's whole job is installing, updating, and launching other OpenAEC desktop apps. A strict-confined snap is sandboxed and generally cannot invoke arbitrary other executables outside its own snap, write anywhere it wants on the filesystem, or manage other installed packages — which may conflict with the installer's core purpose, depending on exactly how it does the installing/launching today (its package.json/Cargo.toml only declare plugin-opener + plugin-store, so the actual install/launch logic likely lives in custom Rust code I haven't audited). Recommend testing the real installed-snap behavior (not just "does it launch") before publishing to stable — if installing/launching other apps doesn't work under strict confinement, confinement: classic is the alternative, but that requires manual Snap Store review approval on every release (much slower iteration).

Test plan

• Register openaec-installer on snapcraft.io.
• Merge, then run a workflow_dispatch test build against an existing tag (e.g. v0.2.0).
• Install the resulting snap and actually exercise the install/update/launch flow for another OpenAEC app, not just confirm the window opens — this is the part strict confinement could break.
• If that works: add SNAPCRAFT_STORE_CREDENTIALS (via snapcraft export-login --snaps=openaec-installer --channels=stable) and re-run against the real release tag to publish.
• If it doesn't work: revisit confinement: classic before publishing further.

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