Add a Check for updates button under Settings; keep the self edge when settings are saved

Saving settings replaced the whole Settings value, which would have dropped SelfEdgeID and made the
next start create a second self edge with a new token.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

settings.go

@@ -41,6 +41,7 @@ func (s *Server) putSettings(w http.ResponseWriter, r *http.Request, p *principa
 	if s.opts.DashboardURL != "" {
 		in.DashboardURL = s.st.Settings.DashboardURL // pinned by flag/env
 	}
+	in.SelfEdgeID = s.st.Settings.SelfEdgeID // not editable here; losing it would make the next start create a second self edge
 	s.st.Settings = in
 	s.bump() // edges receive the dashboard URL and ACME settings
 	s.mu.Unlock()