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>

third_party/sendspin-go/pkg/sendspin/server.go

@@ -96,6 +96,12 @@ type Server struct {
 	audioSource         AudioSource
 	consecutiveReadErrs int
 
+	// Accumulates bytes-on-wire across one streamAudio rate window
+	// (encoded chunk size, not raw PCM) so the periodic stats line can
+	// report both chunks/sec and kbps. Touched only from the streamAudio
+	// goroutine so no locking needed.
+	lastWindowBytes int64
+
 	mdnsManager *discovery.Manager
 
 	// server-initiated discovery dialer cancel
@@ -554,12 +560,19 @@ func (s *Server) clientWriter(c *ServerClient) {
 	for {
 		select {
 		case msg := <-c.sendChan:
+			queueDepth := len(c.sendChan)
 			switch v := msg.(type) {
 			case []byte:
 				c.conn.SetWriteDeadline(time.Now().Add(writeDeadline))
+				wStart := time.Now()
 				if err := c.conn.WriteMessage(websocket.BinaryMessage, v); err != nil {
+					log.Printf("clientWriter %s: binary write error: %v (queue %d)", c.name, err, queueDepth)
 					return
 				}
+				if wDur := time.Since(wStart); wDur > 5*time.Millisecond || queueDepth > 5 {
+					log.Printf("clientWriter %s: %s for %d bytes (queue %d)",
+						c.name, wDur.Round(time.Microsecond), len(v), queueDepth)
+				}
 			default:
 				data, err := json.Marshal(v)
 				if err != nil {
@@ -567,6 +580,7 @@ func (s *Server) clientWriter(c *ServerClient) {
 				}
 				c.conn.SetWriteDeadline(time.Now().Add(writeDeadline))
 				if err := c.conn.WriteMessage(websocket.TextMessage, data); err != nil {
+					log.Printf("clientWriter %s: text write error: %v", c.name, err)
 					return
 				}
 			}