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>

go.mod

@@ -5,7 +5,7 @@ go 1.27.3
 require (
 	git.veillax.com/Veillax/bamboo v0.1.3
 	git.veillax.com/Veillax/blacktip v0.1.4
-	git.veillax.com/Veillax/reef v0.1.3
+	git.veillax.com/Veillax/reef v0.1.4
 	github.com/emersion/go-msgauth v0.7.0
 	golang.org/x/crypto v0.58.0
 	modernc.org/sqlite v1.61.0