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>

cmd/hammerhead/main.go

@@ -78,6 +78,8 @@ func main() {
 		SelfDomain:       os.Getenv("HAMMERHEAD_SELF_DOMAIN"),
 		SelfTarget:       localAddr(*listen),
 		SelfTunnel:       os.Getenv("HAMMERHEAD_SELF_TUNNEL"),
+		Version:          version,
+		AutoUpdate:       *autoUpdate,
 	})
 	if err != nil {
 		log.Fatal(err)