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/index.html

@@ -39,6 +39,22 @@
     <div id="extract-status"></div>
   </section>
 
+  <section id="wav-section">
+    <h2>WAV Browser</h2>
+    <div class="wav-controls">
+      <input type="text" id="wav-dir" placeholder="WAV directory" value="./output/wavs">
+      <button id="wav-browse-btn">Browse</button>
+    </div>
+    <div id="wav-player" class="wav-player">
+      <div id="wav-now-playing" class="now-playing"></div>
+      <div class="audio-bar">
+        <div class="audio-progress"><div id="wav-progress-fill" class="audio-progress-fill"></div></div>
+        <span id="wav-time">0:00</span>
+      </div>
+      <div id="wav-list" class="wav-list"></div>
+    </div>
+  </section>
+
   <section id="results-section">
     <h2>Results <span id="result-count"></span></h2>
     <div id="search-results"></div>