fix: stamp update-check cooldown only after a successful fetch

[Borisserz]

Summary

Fixes #320.

• UpdateChecker.check() stamped lastChecked before the GitHub request finished. A failed lookup (offline, timeout, bad payload) still started the 30-minute cooldown, so reopening the popover stayed silent even though no release was seen.
• Stamp the cooldown only after a trusted https://github.com release URL is accepted. Inject a fetch double in tests so the suite does not hit the network.
• Add an in-flight guard so overlapping popover opens do not stack concurrent requests once the early stamp is gone.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation
• Other:

Checklist

• swift build and swift test pass locally (1081 tests, 13 skipped, 0 failures)
• PR title and description are written in English
• No copyrighted assets, secrets, or private tooling references are committed (see CONTRIBUTING)
• Tests were added or updated for this change

Test plan

• testFailedCheckDoesNotStartTheCooldown fails when lastChecked is stamped before fetch, then passes
• testSuccessfulCheckStartsTheCooldownAndAppliesTheRelease passes
• testRejectedReleaseUrlDoesNotStartTheCooldown passes
• swift build and swift test locally: 1081 passed, 13 skipped, 0 failed
• CI swift test