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.go

@@ -0,0 +1,31 @@
+package hammerhead
+
+import (
+	"context"
+	"net/http"
+	"time"
+
+	"git.veillax.com/Veillax/reef/selfupdate"
+)
+
+// checkUpdate asks the release server whether a newer Hammerhead exists. It only looks: updating is
+// left to auto-update or to the person running the server.
+func (s *Server) checkUpdate(w http.ResponseWriter, r *http.Request, p *principal) {
+	o := s.opts
+	cfg := selfupdate.Config{Server: o.UpdateServer, Repo: o.UpdateRepo, App: "hammerhead", Current: o.Version}
+	if cfg.Server == "" {
+		cfg.Server = "https://git.veillax.com"
+	}
+	if cfg.Repo == "" {
+		cfg.Repo = "Veillax/hammerhead"
+	}
+	ctx, cancel := context.WithTimeout(r.Context(), 20*time.Second)
+	defer cancel()
+	out := map[string]any{"current": o.Version, "autoUpdate": o.AutoUpdate, "checked": time.Now()}
+	latest, newer, err := selfupdate.Check(ctx, cfg)
+	out["latest"], out["newer"] = latest, newer
+	if err != nil {
+		out["error"] = err.Error()
+	}
+	writeJSON(w, 200, out)
+}