feat: recursive WAV browsing in subdirectories

Handle ListWavs now uses filepath.WalkDir to find .wav files
in all subdirectories. Returns relative paths (e.g. Kicks/kick.wav).
Serves them correctly via /wavs/ prefix.

Fixed: WalkDir swallows root errors differently than ReadDir —
now detects and reports non-existent root directory errors.

Added test for recursive subdirectory traversal.

serve_test.go

@@ -473,6 +473,41 @@ func TestHandleListWavs_ReturnsWavFiles(t *testing.T) {
 	}
 }
 
+func TestHandleListWavs_RecursiveSubdirs(t *testing.T) {
+	tmp := t.TempDir()
+	wavDir := filepath.Join(tmp, "wavs")
+	subDir := filepath.Join(wavDir, "Kicks")
+	if err := os.MkdirAll(subDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	fd, _ := os.Create(filepath.Join(wavDir, "root.wav"))
+	_ = fd.Close()
+	fd2, _ := os.Create(filepath.Join(subDir, "kick1.wav"))
+	_ = fd2.Close()
+
+	req := httptest.NewRequest("GET", "/api/list-wavs?dir="+wavDir, nil)
+	rr := httptest.NewRecorder()
+	handleListWavs(rr, req)
+
+	var files []string
+	if err := json.Unmarshal(rr.Body.Bytes(), &files); err != nil {
+		t.Fatalf("invalid JSON: %v", err)
+	}
+	if len(files) != 2 {
+		t.Errorf("expected 2 wav files, got %d: %v", len(files), files)
+	}
+	found := make(map[string]bool)
+	for _, f := range files {
+		found[f] = true
+	}
+	if !found["root.wav"] {
+		t.Errorf("missing root.wav, got %v", files)
+	}
+	if !found[filepath.Join("Kicks", "kick1.wav")] {
+		t.Errorf("missing Kicks/kick1.wav, got %v", files)
+	}
+}
+
 func TestHandleListWavs_EmptyDir(t *testing.T) {
 	tmp := t.TempDir()
 	req := httptest.NewRequest("GET", "/api/list-wavs?dir="+tmp, nil)
@@ -486,6 +521,9 @@ func TestHandleListWavs_EmptyDir(t *testing.T) {
 	if err := json.Unmarshal(rr.Body.Bytes(), &files); err != nil {
 		t.Fatalf("invalid JSON: %v", err)
 	}
+	if files == nil {
+		t.Fatal("expected empty array [], got null")
+	}
 	if len(files) != 0 {
 		t.Errorf("expected 0 files, got %d", len(files))
 	}