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/internal/server/flac_encoder.go

@@ -24,6 +24,8 @@ type FLACEncoder struct {
 	channels    int
 	bitDepth    int
 	blockSize   int
+	frameNum    uint64
+	chanBufs    [][]int32 // reused per-channel sample buffers
 }
 
 // NewFLACEncoder creates a FLAC encoder with prediction analysis enabled.
@@ -54,6 +56,11 @@ func NewFLACEncoder(sampleRate, channels, bitDepth, blockSize int) (*FLACEncoder
 	copy(codecHeader, buf.Bytes())
 	buf.Reset()
 
+	chanBufs := make([][]int32, channels)
+	for ch := range chanBufs {
+		chanBufs[ch] = make([]int32, blockSize)
+	}
+
 	return &FLACEncoder{
 		encoder:     enc,
 		buf:         buf,
@@ -62,6 +69,7 @@ func NewFLACEncoder(sampleRate, channels, bitDepth, blockSize int) (*FLACEncoder
 		channels:    channels,
 		bitDepth:    bitDepth,
 		blockSize:   blockSize,
+		chanBufs:    chanBufs,
 	}, nil
 }
 
@@ -92,20 +100,18 @@ func (e *FLACEncoder) Encode(samples []int32) ([]byte, error) {
 		shift = uint(24 - e.bitDepth)
 	}
 
-	// De-interleave into per-channel slices.
+	// De-interleave into pre-allocated per-channel buffers.
 	subframes := make([]*frame.Subframe, e.channels)
 	for ch := 0; ch < e.channels; ch++ {
-		channelSamples := make([]int32, e.blockSize)
+		buf := e.chanBufs[ch]
 		for i := 0; i < e.blockSize; i++ {
-			channelSamples[i] = samples[i*e.channels+ch] >> shift
+			buf[i] = samples[i*e.channels+ch] >> shift
 		}
 		subframes[ch] = &frame.Subframe{
 			SubHeader: frame.SubHeader{
-				// PredVerbatim signals the encoder to run prediction analysis
-				// and pick the optimal method (Constant, Fixed, or Verbatim).
 				Pred: frame.PredVerbatim,
 			},
-			Samples:  channelSamples,
+			Samples:  buf,
 			NSamples: e.blockSize,
 		}
 	}
@@ -127,9 +133,11 @@ func (e *FLACEncoder) Encode(samples []int32) ([]byte, error) {
 			SampleRate:        uint32(e.sampleRate),
 			Channels:          channelAssign,
 			BitsPerSample:     uint8(e.bitDepth),
+			Num:               e.frameNum,
 		},
 		Subframes: subframes,
 	}
+	e.frameNum++
 
 	e.buf.Reset()
 	if err := e.encoder.WriteFrame(f); err != nil {