Drop the external demo: the Widgets tab is the demo

The hamlab-demo site, its Widgets-tab button, the demo_url config key and the make publish
target that fed it are all gone, along with the public read-only token it needed. The
example stays in examples/external-demo as something to read. The installer one-liner
already comes from Gitea releases.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/api/server.go

@@ -1013,9 +1013,8 @@ func (s *Server) me(w http.ResponseWriter, r *http.Request) {
 	}
 	_, rebootErr := os.Stat("/run/reboot-required")
 	updateVersion, _ := s.behind()
-	var demoURL, addr string
+	var addr string
 	s.cfg.View(func(c *config.Config) {
-		demoURL = c.DemoURL
 		if h, _, err := net.SplitHostPort(c.Listen); err == nil && h != "" && h != "0.0.0.0" && h != "::" {
 			addr = h // the address this console listens on, for the machine menu
 		}
@@ -1023,7 +1022,7 @@ func (s *Server) me(w http.ResponseWriter, r *http.Request) {
 	jsonOK(w, 200, map[string]any{
 		"host": s.hostName(), "user": s.user, "account": actorOf(r).User, "version": s.version,
 		"load": load, "uptime": uptime, "service_started": s.started,
-		"reboot_needed": rebootErr == nil, "fingerprint": s.fingerprint, "arch": runtime.GOARCH, "demo_url": demoURL, "addr": addr,
+		"reboot_needed": rebootErr == nil, "fingerprint": s.fingerprint, "arch": runtime.GOARCH, "addr": addr,
 		"update_available": updateVersion,
 	})
 }