Surface the Fyne UI's "Download latest version" / "Install version X.Y.Z"
About-submenu entry in the Wails tray. The item starts hidden and is
revealed by onUpdateAvailable when the daemon emits EventUpdateAvailable;
opt-in updates open github.com/netbirdio/netbird/releases/latest in the
browser, enforced updates surface the in-window /update progress page
and call TriggerUpdate on the daemon.
Also lift every user-facing string and external URL in tray.go into
named const declarations at the top of the file, so future copy edits
and (eventual) localisation have a single source of truth.
The /update React route is the frontend counterpart and is owned by the
React side of the refactor.