Support m3u playlist files to define chapter order

The chapter order on the tonie is already fully API-controlled via
SortChaptersOfTonie/PATCH. This adds an explicit way to define that
order via an .m3u/.m3u8 file placed in the tonie folder, instead of
relying on alphabetical filenames:

- internal/syncer/playlist.go: finds a single playlist file in the
  tonie folder, parses its entries (ignoring blanks/#-comments), and
  reorders local tracks accordingly. Tracks not referenced by the
  playlist are appended afterwards so nothing is silently dropped;
  playlist entries without a matching local file are ignored.
  Multiple playlist files in the same folder is an error (ambiguous
  order).
- ListLocalTracks now prefers this playlist-defined order, falling
  back to alphabetical filename order when no playlist is present.
- Tests covering playlist ordering, unreferenced-track handling,
  missing entries, and the multiple-playlists error case.
- README documents the .m3u/.m3u8 convention.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/syncer/playlist_test.go

@@ -0,0 +1,104 @@
+package syncer
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func writeFile(t *testing.T, dir, name, content string) string {
+	t.Helper()
+	p := filepath.Join(dir, name)
+	if err := os.WriteFile(p, []byte(content), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	return p
+}
+
+func TestListLocalTracksUsesM3UOrderWhenPresent(t *testing.T) {
+	dir := t.TempDir()
+	writeFile(t, dir, "b-track.mp3", "data")
+	writeFile(t, dir, "a-track.mp3", "data")
+	writeFile(t, dir, "c-track.mp3", "data")
+	writeFile(t, dir, "playlist.m3u", "#EXTM3U\nb-track.mp3\nc-track.mp3\na-track.mp3\n")
+
+	tracks, err := ListLocalTracks(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := titles(tracks)
+	want := []string{"b-track", "c-track", "a-track"}
+	if !equalStrings(got, want) {
+		t.Fatalf("expected order %v, got %v", want, got)
+	}
+}
+
+func TestListLocalTracksAppendsUnreferencedTracksAfterPlaylist(t *testing.T) {
+	dir := t.TempDir()
+	writeFile(t, dir, "a-track.mp3", "data")
+	writeFile(t, dir, "b-track.mp3", "data")
+	writeFile(t, dir, "z-new-track.mp3", "data")
+	// Playlist doesn't mention the newly added track yet.
+	writeFile(t, dir, "playlist.m3u", "b-track.mp3\na-track.mp3\n")
+
+	tracks, err := ListLocalTracks(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := titles(tracks)
+	want := []string{"b-track", "a-track", "z-new-track"}
+	if !equalStrings(got, want) {
+		t.Fatalf("expected order %v, got %v", want, got)
+	}
+}
+
+func TestListLocalTracksIgnoresMissingPlaylistEntries(t *testing.T) {
+	dir := t.TempDir()
+	writeFile(t, dir, "a-track.mp3", "data")
+	writeFile(t, dir, "playlist.m3u", "does-not-exist.mp3\na-track.mp3\n")
+
+	tracks, err := ListLocalTracks(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := titles(tracks)
+	want := []string{"a-track"}
+	if !equalStrings(got, want) {
+		t.Fatalf("expected order %v, got %v", want, got)
+	}
+}
+
+func TestListLocalTracksFallsBackToAlphabeticalWithoutPlaylist(t *testing.T) {
+	dir := t.TempDir()
+	writeFile(t, dir, "b-track.mp3", "data")
+	writeFile(t, dir, "a-track.mp3", "data")
+
+	tracks, err := ListLocalTracks(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := titles(tracks)
+	want := []string{"a-track", "b-track"}
+	if !equalStrings(got, want) {
+		t.Fatalf("expected order %v, got %v", want, got)
+	}
+}
+
+func TestListLocalTracksErrorsOnMultiplePlaylists(t *testing.T) {
+	dir := t.TempDir()
+	writeFile(t, dir, "a-track.mp3", "data")
+	writeFile(t, dir, "one.m3u", "a-track.mp3\n")
+	writeFile(t, dir, "two.m3u8", "a-track.mp3\n")
+
+	if _, err := ListLocalTracks(dir); err == nil {
+		t.Fatal("expected error when multiple playlist files are present")
+	}
+}
+
+func titles(tracks []Track) []string {
+	titles := make([]string, len(tracks))
+	for i, t := range tracks {
+		titles[i] = t.Title
+	}
+	return titles
+}