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>

store.go

@@ -180,6 +180,14 @@ type Options struct {
 	SelfDomain string
 	SelfTarget string
 	SelfTunnel string
+
+	// Version is the running release (e.g. v0.1.5), shown under Settings with a check for updates.
+	Version string
+	// AutoUpdate says whether this process updates itself (HAMMERHEAD_AUTO_UPDATE), for the same screen.
+	AutoUpdate bool
+	// UpdateServer and UpdateRepo say where releases are published; the defaults are this project's own.
+	UpdateServer string
+	UpdateRepo   string
 }
 
 type Server struct {