fix: initialize files as empty slice instead of nil var

var files []string marshals to JSON null, breaking the frontend
which expects an array. Using files := []string{} ensures [].

serve.go

@@ -279,7 +279,7 @@ func handleListWavs(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	var files []string
+	files := []string{}
 	for _, e := range entries {
 		if !e.IsDir() && strings.HasSuffix(strings.ToLower(e.Name()), ".wav") {
 			files = append(files, e.Name())