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>

selfedge_test.go

@@ -96,3 +96,26 @@ func TestNoSelfEdgeUnlessAsked(t *testing.T) {
 		t.Fatal("token file written without HAMMERHEAD_SELF_DOMAIN")
 	}
 }
+
+func TestSavingSettingsKeepsTheSelfEdge(t *testing.T) {
+	dir := t.TempDir()
+	opts := Options{AdminEmail: "admin@test.local", AdminPassword: "password123", MasterKey: key, SelfDomain: "hh.example.com", SelfTarget: "127.0.0.1:8080"}
+	s, err := New(dir, opts)
+	if err != nil {
+		t.Fatal(err)
+	}
+	token := tokenFile(t, dir)
+	do := apiClient(t, s)
+	if code, b := do("PUT", "/api/admin/settings", map[string]any{"dashboardUrl": "https://hh.example.com", "acmeEmail": "me@example.com"}); code != 200 {
+		t.Fatalf("settings: %d %s", code, b)
+	}
+	s.Close()
+	s2, err := New(dir, opts)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer s2.Close()
+	if len(s2.st.Edges) != 1 || tokenFile(t, dir) != token {
+		t.Fatalf("saving settings lost the self edge: %d edges", len(s2.st.Edges))
+	}
+}