feat: the app checks GitHub for a newer release and says so in the title bar

[ba2slk]

Termspace asks the GitHub releases API for a newer release at startup and once every 24 hours: one anonymous GET, nothing else sent. When one exists, a chip at the end of the title bar's left group names the version and opens the release page on click. Nothing is downloaded; installing is the user's, on both platforms, as it is today.

Dismissing the chip hides it for the run. A stable build sees only stable releases; a prerelease build sees prereleases too, and never an older stable. A version higher than anything published (a local build) reports nothing. Drafts, unparseable tags and non-github.com URLs are skipped.

updateCheck (default 1) in settings.yaml or the toggle in Settings › Updates turns the automatic check off. "Check now" in the same section runs regardless and reports inline (up to date / version available with a button to the page / GitHub unreachable). The renderer only ever receives a state and a version string; the URL stays in main. Three IPC channels: update:check, update:open-release, update:state. Not started under the self-check.

update-check.ts (SemVer, candidate choice) and updater.ts (timer, injected fetch) are pure and unit tested; the chip and the settings rows have DOM tests. Checked by hand against the live API by running a build versioned as 1.1.0-beta.3: the chip named 1.1.0-beta.4 and the settings row did the same. README, MANUAL-QA and the engineering notes cover it; in-place AppImage replacement is deferred and noted there.

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