refactor: delete dead code, dedupe helpers, add tests for untested packages

Dead code
  Removed, each verified to have no non-test caller: promptSearchSelection
  (77 lines, superseded by promptSearchSelectionMenu), AddPlaylistByTrackIDs,
  download.NewWithVerifySSL, qobuz.Client.GetTrackMetadata,
  tidal.Client.trackSupportsAtmos (test-only, and the comment above
  GetDownloadable already documented its removal from the live path) and its
  now-orphaned test, yandex.canonicalTrackRequestID (a no-op returning its
  trimmed input; the call site uses strings.TrimSpace directly) and the unused
  defaultEstimatedKbps constant.

Config keys that did nothing
  qobuz.download_booklets and misc.check_for_updates were declared, defaulted
  and documented in config.toml.example but read by no code. The whole
  [youtube] block went too: there is no YouTube provider. go-toml ignores
  unknown keys, verified, so existing user configs still load.

Duplicated helpers
  tidal.stringify and yandex.firstNonEmpty were byte-identical copies of
  jsonutil.StringFromAny and jsonutil.FirstNonEmpty. displayArtistNames
  existed three times, byte-identical, in internal/app, provider/yandex and
  provider/beatport; it now lives in jsonutil. The CLI's normalizeCodec
  delegates to config.NormalizeCodec so the flag and the config file are
  checked by the same rules.

Tests for packages that had none
  internal/jsonutil (0% -> 100%) and internal/netutil (0% -> 78%), the latter
  covering redactURL, which is a security control, and the zero-timeout
  contract that internal/download and the qobuz body copy depend on.
  internal/config rose to 82% with coverage of the new Validate(), the
  0600 permissions, CloneConfigData not sharing slices, and a concurrent
  UpdateTokens test that exercises the mutex under -race.

Test suite is ~85x faster
  Provider tests built clients with the default 60 requests/minute, so the
  rate limiter slept 1s per HTTP call: deezer took 31s, tidal 11s, qobuz 8s.
  Each provider test package now builds its config through a testConfigData
  helper that disables throttling. Whole suite: ~60s -> 0.7s, and 2.8s
  under -race.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/app/app.go

@@ -298,47 +298,6 @@ func (m *Main) AddByID(ctx context.Context, source, mediaType, id string) error
 	return nil
 }
 
-func (m *Main) AddPlaylistByTrackIDs(ctx context.Context, source, playlistID, playlistName string, trackIDs []string) error {
-	p, err := m.GetLoggedInProvider(ctx, source)
-	if err != nil {
-		return err
-	}
-	if strings.TrimSpace(playlistName) == "" {
-		playlistName = playlistID
-	}
-	ids := make([]string, 0, len(trackIDs))
-	for _, id := range trackIDs {
-		id = strings.TrimSpace(id)
-		if id != "" {
-			ids = append(ids, id)
-		}
-	}
-	if len(ids) == 0 {
-		return fmt.Errorf("playlist %q has no track ids", playlistName)
-	}
-
-	pending := media.PendingFunc{
-		ResolveFn: func(context.Context) (media.Media, error) {
-			metaItems := make([]any, 0, len(ids))
-			for _, id := range ids {
-				metaItems = append(metaItems, map[string]any{"id": id})
-			}
-			playlistMeta := map[string]any{
-				"name":   playlistName,
-				"tracks": map[string]any{"items": metaItems},
-			}
-			return media.MediaFunc{RipFn: func(ctx context.Context) error {
-				if m.Config.Session.CLI.TextOutput {
-					m.logf("Downloading playlist: %s\n", playlistName)
-				}
-				return m.ripPlaylist(ctx, p, source, playlistID, playlistMeta)
-			}}, nil
-		},
-	}
-	m.Pending = append(m.Pending, pending)
-	return nil
-}
-
 func (m *Main) AddMixedPlaylistByTrackRefs(ctx context.Context, playlistID, playlistName string, refs []PlaylistTrackRef) error {
 	if strings.TrimSpace(playlistName) == "" {
 		playlistName = playlistID
@@ -1728,11 +1687,11 @@ func extractAlbumArtist(albumMeta map[string]any) string {
 			}
 		}
 		if len(names) > 0 {
-			return displayArtistNames(names)
+			return jsonutil.DisplayArtistNames(names)
 		}
 	}
 	if names := stringSliceFromAny(albumMeta["artist_names"]); len(names) > 0 {
-		return displayArtistNames(names)
+		return jsonutil.DisplayArtistNames(names)
 	}
 	artist := jsonutil.NestedString(albumMeta, "artist", "name")
 	if artist != "" {
@@ -1740,16 +1699,3 @@ func extractAlbumArtist(albumMeta map[string]any) string {
 	}
 	return "Unknown"
 }
-
-func displayArtistNames(names []string) string {
-	switch len(names) {
-	case 0:
-		return ""
-	case 1:
-		return names[0]
-	case 2:
-		return names[0] + " & " + names[1]
-	default:
-		return strings.Join(names[:len(names)-1], ", ") + " & " + names[len(names)-1]
-	}
-}