fix: prevent Play() deadlock when device is not open

Play() held mu then called SwitchDevice() which also tried to acquire mu,
causing a deadlock. Replace with direct call to openDeviceLocked() which
does not lock.

Add regression test that verifies Play() returns an error (not deadlock)
when called on an uninitialized mixer.

internal/mixer/mixer.go

@@ -112,14 +112,15 @@ func Play(path string, loopForever bool, cfg *db.AppConfig) error {
 	defer mu.Unlock()
 
 	// Check if mixer has been initialized and that a device is open
-	if !inited || !deviceOpen {
-		if !deviceOpen {
-			SwitchDevice("")             // Re-switch to the default device
-			Play(path, loopForever, cfg) // Re-run the function
-		}
+	if !inited {
 		logger.Error("audio mixer not initialized")
 		return errors.New("audio mixer not initialized")
 	}
+	if !deviceOpen {
+		if err := openDeviceLocked(""); err != nil {
+			return fmt.Errorf("failed to open audio device: %w", err)
+		}
+	}
 	if path == "" {
 		logger.Error("empty audio path")
 		return errors.New("empty audio path")