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

web/ui/style.css

@@ -205,4 +205,109 @@ h2 {
   margin-top: 10px;
   font-size: 13px;
   color: var(--text2);
+}
+
+.wav-controls {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+}
+.wav-controls input {
+  flex: 1;
+  min-width: 200px;
+  padding: 10px 14px;
+  border: 1px solid var(--bg3);
+  border-radius: var(--radius);
+  background: var(--bg);
+  color: var(--text);
+  font-size: 14px;
+}
+.wav-controls button {
+  padding: 10px 20px;
+  border: none;
+  border-radius: var(--radius);
+  background: var(--accent);
+  color: white;
+  font-size: 14px;
+  cursor: pointer;
+}
+
+.wav-player {
+  margin-top: 14px;
+}
+
+.now-playing {
+  font-size: 13px;
+  color: var(--green);
+  margin-bottom: 6px;
+  min-height: 18px;
+}
+
+.audio-bar {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  margin-bottom: 10px;
+}
+
+.audio-progress {
+  flex: 1;
+  height: 6px;
+  background: var(--bg3);
+  border-radius: 3px;
+  overflow: hidden;
+}
+
+.audio-progress-fill {
+  height: 100%;
+  background: var(--green);
+  border-radius: 3px;
+  width: 0%;
+  transition: width 0.1s;
+}
+
+#wav-time {
+  font-size: 12px;
+  color: var(--text2);
+  min-width: 40px;
+}
+
+.wav-list {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
+  gap: 8px;
+}
+
+.wav-item {
+  background: var(--bg);
+  border: 1px solid var(--bg3);
+  border-radius: var(--radius);
+  padding: 10px 12px;
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  cursor: pointer;
+}
+.wav-item:hover {
+  border-color: var(--accent);
+}
+.wav-item.playing {
+  border-color: var(--green);
+  background: rgba(78, 204, 163, 0.1);
+}
+.wav-item .wav-name {
+  flex: 1;
+  font-size: 13px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.wav-item .wav-play-btn {
+  background: var(--accent2);
+  color: white;
+  border: none;
+  border-radius: 4px;
+  padding: 4px 10px;
+  font-size: 11px;
+  cursor: pointer;
 }