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

util_test.go

@@ -99,6 +99,8 @@ func TestFindAkaiutil(t *testing.T) {
 	}()
 
 	t.Run("env var set to missing file", func(t *testing.T) {
+		tmp := t.TempDir()
+		t.Chdir(tmp)
 		_ = os.Setenv("AKAIUTIL", "/nonexistent/akaiutil")
 		os.Args[0] = "/some/binary"
 		_, err := findAkaiutil()
@@ -109,6 +111,7 @@ func TestFindAkaiutil(t *testing.T) {
 
 	t.Run("env var set to existing file", func(t *testing.T) {
 		tmp := t.TempDir()
+		t.Chdir(tmp)
 		f := tmp + "/myutil"
 		fd, _ := os.Create(f)
 		defer func() { _ = fd.Close() }()
@@ -124,6 +127,7 @@ func TestFindAkaiutil(t *testing.T) {
 
 	t.Run("finds binary next to argv[0]", func(t *testing.T) {
 		tmp := t.TempDir()
+		t.Chdir(tmp)
 		exe := tmp + "/mybinary"
 		fd, _ := os.Create(exe)
 		defer func() { _ = fd.Close() }()
@@ -143,6 +147,7 @@ func TestFindAkaiutil(t *testing.T) {
 
 	t.Run("finds binary in third_party relative to argv[0]", func(t *testing.T) {
 		tmp := t.TempDir()
+		t.Chdir(tmp)
 		exe := tmp + "/somebin"
 		fd, _ := os.Create(exe)
 		defer func() { _ = fd.Close() }()
@@ -166,6 +171,7 @@ func TestFindAkaiutil(t *testing.T) {
 
 	t.Run("env var takes priority over path candidates", func(t *testing.T) {
 		tmp := t.TempDir()
+		t.Chdir(tmp)
 		envFile := tmp + "/envutil"
 		pathFile := tmp + "/pathutil"
 		fd1, _ := os.Create(envFile)
@@ -185,6 +191,7 @@ func TestFindAkaiutil(t *testing.T) {
 
 	t.Run("no akaiutil found returns error", func(t *testing.T) {
 		tmp := t.TempDir()
+		t.Chdir(tmp)
 		exe := tmp + "/nobinaryhere"
 		fd, _ := os.Create(exe)
 		defer func() { _ = fd.Close() }()