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/client_dialer.go

@@ -128,27 +128,23 @@ func (d *clientDialer) claim(instance string) bool {
 }
 
 // release updates the instance slot after a dial attempt completes.
-// On success (dialErr == nil) the instance stays latched in the active
-// set — we never re-dial a successfully-connected instance. On error
-// it frees the slot and schedules an exponentially-backed-off cooldown
-// before the next retry.
+// The slot is always freed — dial() only returns after handleConnection
+// has returned, so the connection is already dead by the time we get
+// here, and a future mDNS re-emission should be free to redial. On
+// error it also schedules an exponentially-backed-off cooldown before
+// the next retry.
 func (d *clientDialer) release(instance string, dialErr error) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
 
+	delete(d.active, instance)
+
 	if dialErr == nil {
-		// Latch: keep active[instance] = true so future discovery
-		// events for this instance are silently ignored. The server
-		// already handled the connection; re-dialing would create a
-		// duplicate session.
 		delete(d.failures, instance)
 		delete(d.cooldown, instance)
 		return
 	}
 
-	// Dial failed — release the slot so the instance can be retried
-	// after the backoff period.
-	delete(d.active, instance)
 	d.failures[instance]++
 	backoff := d.baseBackoff * time.Duration(1<<(d.failures[instance]-1))
 	if backoff > d.maxBackoff || backoff <= 0 {