New "Read Release Notes" entries in the Help menu (current installed
version) and the "Check for Updates" alert (the new, not-yet-installed
version) render a release's markdown notes in a Confab modal.
- UpdateCheckService.fetchReleaseNotes(forTag:) fetches a release's
title + body from Gitea's public releases-by-tag API, caching the
result by version tag in the settings table (a published release's
notes don't change, so no need to refetch on every view).
- ReleaseNotesView reuses the existing MarkdownContentView renderer;
shows a friendly "not available yet" state for versions with no
published Gitea release (e.g. a dev build ahead of the last release).
- ReleaseNotesRequest carries which version to show atomically via
.sheet(item:), per this project's established sheet-timing pattern.
- Removed the redundant "Release Page" button from the update alert
now that notes show in-app; added an explicit .keyboardShortcut
(.cancelAction) to its cancel button so Escape actually closes it —
role: .cancel alone didn't do it, since NSAlert only auto-binds
Escape to a button literally titled "Cancel".
