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>

internal/live/manager.go

@@ -186,9 +186,11 @@ func (m *Manager) SetActivePreset(id string) error {
 		return nil
 	}
 
-	// On entry from off, default to paused; otherwise carry over.
+	// On entry from off, auto-start playback so picking a preset is a
+	// one-click action. Switching between presets carries over the
+	// current playback state.
 	if prev == "" {
-		m.playback = Paused
+		m.playback = Playing
 	}
 
 	m.src.SetTarget(nextSource)