Securexe Launcher v0.1.16

Add a real one-click updater for the launcher itself (macOS)

The update notification's button previously just opened the GitHub
release page, leaving the actual install (download DMG, mount, drag to
Applications, eject, relaunch) entirely manual. This runs that whole
sequence via a visible Terminal script instead: download, quit the
running instance, install, strip the quarantine flag, relaunch.

Deliberately not a "real" auto-updater — the app is ad-hoc signed (no
paid Apple Developer ID yet), and per Apple's own DTS guidance, ad-hoc
signatures have no stable identity across builds, so TCC permission
grants (Local Network, etc.) will still reset on update regardless of
how the new build gets installed. Stripping quarantine here does fix
the Gatekeeper "unidentified developer" prompt specifically (the
standard workaround for that), but the permission-reset behavior is a
real limitation that only a real Developer ID + notarization removes —
documented in the code rather than glossed over.

Windows/Linux fall back to opening the release page, same as before —
no scripted installer for those yet.

Verified: the generated script is checked for syntax validity (`sh -n`,
real parse-check, not just string matching) and correct step ordering
in a test — never executed for real, since it operates on
/Applications and would kill/replace whatever's actually installed.