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_stream.go

@@ -18,6 +18,8 @@ func (s *Server) streamAudio() {
 
 	tickBudget := time.Duration(ChunkDurationMs) * time.Millisecond
 	var lastTick time.Time
+	var chunkCount int
+	rateWindowStart := time.Now()
 	for {
 		select {
 		case t := <-ticker.C:
@@ -35,6 +37,21 @@ func (s *Server) streamAudio() {
 			lastTick = t
 
 			s.generateAndSendChunk()
+			chunkCount++
+
+			if elapsed := time.Since(rateWindowStart); elapsed >= 10*time.Second {
+				rate := float64(chunkCount) / elapsed.Seconds()
+				avgBytes := 0
+				if chunkCount > 0 {
+					avgBytes = int(s.lastWindowBytes) / chunkCount
+				}
+				kbps := float64(s.lastWindowBytes) * 8 / 1000.0 / elapsed.Seconds()
+				log.Printf("ENGINE STATS: %.3f chunks/sec, avg %d bytes/chunk, %.1f kbps over %s (%d chunks)",
+					rate, avgBytes, kbps, elapsed.Round(time.Millisecond), chunkCount)
+				chunkCount = 0
+				s.lastWindowBytes = 0
+				rateWindowStart = time.Now()
+			}
 		case <-s.stopChan:
 			log.Printf("Audio streaming stopping")
 			return
@@ -100,6 +117,10 @@ func (s *Server) generateAndSendChunk() {
 		tracker := c.bufferTracker
 		c.mu.RUnlock()
 
+		if codec == "" {
+			continue
+		}
+
 		switch codec {
 		case "opus":
 			if opusEncoder != nil {
@@ -139,6 +160,7 @@ func (s *Server) generateAndSendChunk() {
 		}
 
 		chunk := CreateAudioChunk(playbackTime, audioData)
+		s.lastWindowBytes += int64(len(chunk))
 
 		if tracker != nil {
 			chunkDurationUs := int64(ChunkDurationMs) * 1000