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>

server.go

@@ -97,6 +97,7 @@ func (s *Server) Handler() http.Handler {
 	mux.HandleFunc("GET /api/admin/settings", s.withServerAdmin(s.getSettings))
 	mux.HandleFunc("PUT /api/admin/settings", s.withServerAdmin(s.putSettings))
 	mux.HandleFunc("POST /api/admin/settings/test-mail", s.withServerAdmin(s.testMail))
+	mux.HandleFunc("GET /api/admin/update", s.withServerAdmin(s.checkUpdate))
 	mux.HandleFunc("GET /api/admin/audit", s.withServerAdmin(func(w http.ResponseWriter, r *http.Request, p *principal) {
 		writeJSON(w, 200, s.logs.queryAudit("", 300, 0))
 	}))