Fix ESP32 audio dropouts and improve streaming reliability

FLAC encoder now tracks frame numbers and reuses per-channel sample
buffers to reduce allocations. Client dialer always frees the active
slot on release so mDNS re-emissions can reconnect cleanly. Server
send buffer uses drop-oldest instead of hard failure, with lateness
tracking. Added engine stats logging (chunks/sec, kbps) and writer
diagnostics. UI simplified to a single play/pause toggle that
auto-starts playback on preset selection. Added --preset CLI flag
for headless startup. Removed stale systemd service files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/live-server/main.go

@@ -60,6 +60,7 @@ func main() {
 		mqttDiscovery string
 		rate          int
 		channels      int
+		preset        string
 	)
 
 	addInt(&port, 8927, "port", "p", "Sendspin WebSocket port")
@@ -74,6 +75,7 @@ func main() {
 	addStr(&mqttDiscovery, "homeassistant", "mqtt-discovery", "", "Home Assistant discovery prefix")
 	addInt(&rate, 48000, "rate", "r", "capture sample rate")
 	addInt(&channels, 2, "channels", "", "capture channels")
+	addStr(&preset, "", "preset", "", "activate this preset id at startup (begins playback immediately)")
 
 	flag.Usage = usage
 	flag.Parse()
@@ -147,6 +149,13 @@ func main() {
 	go func() { serverDone <- server.Start() }()
 	log.Printf("Sendspin server %q on :%d", name, port)
 
+	if preset != "" {
+		if err := mgr.SetActivePreset(preset); err != nil {
+			log.Fatalf("activate preset %q: %v", preset, err)
+		}
+		log.Printf("preset %q activated at startup", preset)
+	}
+
 	sig := make(chan os.Signal, 1)
 	signal.Notify(sig, os.Interrupt, syscall.SIGTERM)
 	select {