feat(screenshots): add headless-Chrome capture pipeline with fake speaker

Refreshes docs/images/ui-{settings,devices,sync,migration}.png by
driving the web UI in chromedp against a synthetic speaker, so
documentation can be regenerated without real hardware and without
leaking personal data from the local network.

Three independent pieces:

- pkg/service/testing/fakespeaker — embeddable library serving the
  HTTP and telnet surface the migration wizard probes (/info,
  /presets, /recents and a getpdo CurrentSystemConfiguration reply
  that places the device on the unmigrated happy path).
- cmd/dummy-speaker — thin CLI wrapping the library; self-registers
  with a running service via POST /setup/devices.
- scripts/screenshots — chromedp runner driven by a JSON manifest;
  decoupled from speaker/service setup so it can target any backend
  URL. run.sh orchestrates a one-shot end-to-end capture and seeds
  settings.json with a generic hostname plus discovery disabled to
  keep real-network state out of the captures.

Captures are at DPR=2 for retina-sharp text.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/dummy-speaker/main.go

@@ -0,0 +1,107 @@
+// Command dummy-speaker runs an HTTP-only fake SoundTouch speaker and
+// optionally registers it with a running soundtouch-service so the web UI
+// has a device to display.
+//
+// Intended for documentation screenshots and local UI smoke checks. Do not
+// use against a real network — the fixture payload is synthetic and would
+// confuse other tooling that expects live device data.
+//
+// Example:
+//
+//	dummy-speaker --port 8090 --register http://localhost:8000
+package main
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"github.com/gesellix/bose-soundtouch/pkg/service/testing/fakespeaker"
+)
+
+func main() {
+	listen := flag.String("listen", "127.0.0.1:8090", "bind address for the fake speaker's HTTP API")
+	telnetListen := flag.String("telnet-listen", "127.0.0.1:17000", "bind address for the fake speaker's telnet diagnostic shell (empty to disable)")
+	register := flag.String("register", "", "service base URL (e.g. http://localhost:8000) to self-register with via POST /setup/devices")
+	registerAs := flag.String("register-as", "", "address to send to /setup/devices (defaults to --listen)")
+
+	flag.Parse()
+
+	s, err := fakespeaker.Start(fakespeaker.Config{
+		HTTPListen:   *listen,
+		TelnetListen: *telnetListen,
+	})
+	if err != nil {
+		log.Fatalf("start fake speaker: %v", err)
+	}
+
+	log.Printf("fake speaker HTTP listening on http://%s", s.HTTPAddr())
+
+	if addr := s.TelnetAddr(); addr != "" {
+		log.Printf("fake speaker telnet listening on tcp://%s", addr)
+	}
+
+	if *register != "" {
+		target := *registerAs
+		if target == "" {
+			target = s.HTTPAddr()
+		}
+
+		if err := registerWithService(*register, target); err != nil {
+			log.Printf("self-register failed: %v (continuing anyway)", err)
+		} else {
+			log.Printf("registered %s with service at %s", target, *register)
+		}
+	}
+
+	sig := make(chan os.Signal, 1)
+	signal.Notify(sig, syscall.SIGINT, syscall.SIGTERM)
+	<-sig
+
+	log.Printf("shutting down")
+
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	if err := s.Stop(ctx); err != nil {
+		log.Printf("stop: %v", err)
+	}
+}
+
+func registerWithService(serviceURL, deviceAddr string) error {
+	body, err := json.Marshal(map[string]string{"ip": deviceAddr})
+	if err != nil {
+		return err
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, serviceURL+"/setup/devices", bytes.NewReader(body))
+	if err != nil {
+		return err
+	}
+
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return err
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode >= 300 {
+		return fmt.Errorf("service responded %s", resp.Status)
+	}
+
+	return nil
+}