Backspace 1.0.6

Backspace 1.0.6

Backspace now has Flatpak packaging, contributed by the community, and the
contributing guide says what a packaging or platform change needs to bring
with it.

What changed

Flatpak packaging

The desktop app can now be built as a Flatpak. The manifest, the offline
dependency sources, the AppStream metadata and a CI job that builds it on
x86_64 and aarch64 are all in the repository, and every tagged release now
regenerates and validates that metadata automatically.

In the Flatpak build, updates are Flatpak's job. The app detects the sandbox
and hides its own update and auto-launch controls instead of offering actions
it cannot perform, the same way the ad-hoc signed macOS build learned to do in
1.0.5.

This is the groundwork. A Flathub listing needs the published manifest to
point at a tagged release, which this release is, so the submission can
follow.

Thank you to st7105 and gr1n-the-big-floppa for the packaging work and for two
rounds of careful fixes during review.

Contributing guide

CONTRIBUTING.md now explains what a reviewable pull request looks like: a
size the maintainers can actually read, and, for packaging, platform and UI
changes, evidence that the change was run rather than only built. The pull
request template asks for the same.

Release automation

The first run of the new Flatpak release job failed on a git ownership check
inside its build container. That is fixed, and this is the release that
proved it.

Upgrading

Server operators: ./update.sh as usual. Desktop users on Windows and Linux
get the update in the app; macOS users download the new build by hand, as
before.