feat: Audio Preview Player — browse and play WAV samples via Web Audio API

Issue: #9

Backend:
- GET /api/list-wavs?dir=<path> — lists .wav files in a directory
- GET /wavs/<file> — static file serving from ./output/wavs

Web UI:
- WAV Browser section with path input + Browse button
- AudioContext-based playback (zero deps) — progress bar, time display
- WAV file grid with Play buttons, active item highlighting

Tests:
- 3 new tests for handleListWavs (wav files, empty dir, nonexistent dir)
- Fix TestFindAkaiutil: add t.Chdir(tmp) so relative path lookups don't
  accidentally find the repo's third_party/akaiutil/akaiutil binary

serve.go

@@ -54,10 +54,20 @@ func cmdServe(args []string) {
 
 	mux.HandleFunc("/api/search", handleSearch)
 	mux.HandleFunc("/api/list", handleList)
+	mux.HandleFunc("/api/list-wavs", handleListWavs)
 	mux.HandleFunc("/api/download", handleAPIDownload)
 	mux.HandleFunc("/api/extract", handleAPIExtract)
 	mux.HandleFunc("/api/progress", handleProgress)
 
+	wavsBase := "./output/wavs"
+	if abs, err := filepath.Abs(wavsBase); err == nil {
+		wavsBase = abs
+	}
+	if err := os.MkdirAll(wavsBase, 0755); err != nil {
+		fatal("create wavs dir: %v", err)
+	}
+	mux.Handle("/wavs/", http.StripPrefix("/wavs/", http.FileServer(http.Dir(wavsBase))))
+
 	addr := fmt.Sprintf("%s:%d", host, port)
 	listener, err := net.Listen("tcp", addr)
 	if err != nil {
@@ -254,6 +264,30 @@ func handleAPIDownload(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, map[string]any{"jobs": jobIDs})
 }
 
+func handleListWavs(w http.ResponseWriter, r *http.Request) {
+	dir := r.URL.Query().Get("dir")
+	if dir == "" {
+		dir = "./output/wavs"
+	}
+	if abs, err := filepath.Abs(dir); err == nil {
+		dir = abs
+	}
+
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		writeJSON(w, map[string]any{"error": err.Error()})
+		return
+	}
+
+	var files []string
+	for _, e := range entries {
+		if !e.IsDir() && strings.HasSuffix(strings.ToLower(e.Name()), ".wav") {
+			files = append(files, e.Name())
+		}
+	}
+	writeJSON(w, files)
+}
+
 func handleProgress(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", "text/event-stream")
 	w.Header().Set("Cache-Control", "no-cache")