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>

update_test.go

@@ -0,0 +1,54 @@
+package hammerhead
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestCheckForUpdates(t *testing.T) {
+	gitea := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/api/v1/repos/Veillax/hammerhead/releases/latest" {
+			http.NotFound(w, r)
+			return
+		}
+		fmt.Fprint(w, `{"tag_name":"v0.3.0","prerelease":false,"assets":[]}`)
+	}))
+	defer gitea.Close()
+	for _, c := range []struct {
+		version string
+		newer   bool
+	}{{"v0.2.0", true}, {"v0.3.0", false}, {"dev", false}} {
+		s, err := New(t.TempDir(), Options{AdminEmail: "admin@test.local", AdminPassword: "password123", MasterKey: key,
+			Version: c.version, UpdateServer: gitea.URL, UpdateRepo: "Veillax/hammerhead", AutoUpdate: true})
+		if err != nil {
+			t.Fatal(err)
+		}
+		do := apiClient(t, s)
+		code, b := do("GET", "/api/admin/update", nil)
+		var out struct {
+			Current, Latest, Error string
+			Newer, AutoUpdate      bool
+		}
+		json.Unmarshal(b, &out)
+		if code != 200 || out.Current != c.version || out.Latest != "v0.3.0" || out.Newer != c.newer || !out.AutoUpdate || out.Error != "" {
+			t.Errorf("%s: %d %s", c.version, code, b)
+		}
+		s.Close()
+	}
+
+	// an unreachable release server is reported, not fatal
+	s, err := New(t.TempDir(), Options{AdminEmail: "admin@test.local", AdminPassword: "password123", MasterKey: key, Version: "v0.1.0", UpdateServer: "http://127.0.0.1:1"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer s.Close()
+	code, b := apiClient(t, s)("GET", "/api/admin/update", nil)
+	var out struct{ Error string }
+	json.Unmarshal(b, &out)
+	if code != 200 || out.Error == "" {
+		t.Errorf("unreachable server: %d %s", code, b)
+	}
+}