feat: cw decoder

app.go

@@ -378,6 +378,8 @@ type App struct {
 	ubFollowStop        chan struct{}     // stops the "follow frequency" loop; nil when off
 	audioMgr            *audio.Manager
 	qsoRec              *audio.Recorder // continuous QSO recorder (rolling pre-roll)
+	cwMu                sync.Mutex      // guards the CW decoder lifecycle
+	cwStop              chan struct{}   // stops the CW decoder capture loop; nil when off
 	dvkRecSlot          int             // slot currently being recorded (DVKStartRecord → DVKStopRecord)
 	dvkPttKeyed         bool            // we keyed PTT for a voice message; unkey when it ends
 	pttMu               sync.Mutex