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.go

@@ -273,18 +273,30 @@ func handleListWavs(w http.ResponseWriter, r *http.Request) {
 		dir = abs
 	}
 
-	entries, err := os.ReadDir(dir)
+	files := []string{}
+	var walkErr error
+	err := filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			if path == dir {
+				walkErr = err
+				return filepath.SkipAll
+			}
+			return nil
+		}
+		if !d.IsDir() && strings.HasSuffix(strings.ToLower(d.Name()), ".wav") {
+			rel, _ := filepath.Rel(dir, path)
+			files = append(files, rel)
+		}
+		return nil
+	})
+	if walkErr != nil {
+		writeJSON(w, map[string]any{"error": walkErr.Error()})
+		return
+	}
 	if err != nil {
 		writeJSON(w, map[string]any{"error": err.Error()})
 		return
 	}
-
-	files := []string{}
-	for _, e := range entries {
-		if !e.IsDir() && strings.HasSuffix(strings.ToLower(e.Name()), ".wav") {
-			files = append(files, e.Name())
-		}
-	}
 	writeJSON(w, files)
 }