In-app BEAST core update notifier (read latest release tag)

[alexeid]

Spin-off from #80. Umbrella: #34.

Background

While scoping #80 (package upgrade support), the original phrasing "upgrade BEAST.base and BEAST.fx" was found to conflate two different things:

• Third-party installed packages — handled by the package manager. Tracked in v2.7→v2.8 parity: package upgrade support (BEAST.base, BEAST.fx, installed packages) #80.
• BEAST core itself (which includes BEAST.base and beast.fx as JPMS modules of the beast3 jar) — upgrading this means replacing the .dmg / .exe / .tar.gz. That's an installer concern, not a package-manager concern.

There's no in-app affordance for the second case. v2.7 users could upgrade BEAST core as a "package"; in v2.8 they have to know to go check beast2.org or the GitHub releases page.

Proposal

On BEAUti / AppLauncher startup (governed by the existing package.update.status property in PackageManager.java:1750), check the latest BEAST 3 release version, compare to the running version, and show a one-shot notice with links to beast2.org and the GitHub releases page when newer. No auto-install — just a pointer.

Source of the latest version: GitHub releases API for CompEvol/beast3 is the obvious data source even if the user-facing link is beast2.org.

Acceptance

• A user running an outdated BEAST core sees one notice (dismissable / opt-out via the same property as package update checks) telling them a newer release is available, with links to beast2.org and the GitHub release page.
• Behaviour respects package.update.status (AUTO_CHECK, DO_NOT_CHECK, AUTO_UPDATE — the latter degrades to AUTO_CHECK for core since we don't auto-install).