fix: remove unused FRAME_SAMPLES_20MS constant in desktop engine

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src-tauri/src/engine.rs

@@ -13,7 +13,6 @@ use wzp_client::audio_io::{AudioCapture, AudioPlayback};
 use wzp_client::call::{CallConfig, CallEncoder};
 use wzp_proto::{CodecId, MediaTransport, QualityProfile};
 
-const FRAME_SAMPLES_20MS: usize = 960;
 const FRAME_SAMPLES_40MS: usize = 1920;
 
 /// Resolve a quality string from the UI to a QualityProfile.