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  (77 lines, superseded by promptSearchSelectionMenu), AddPlaylistByTrackIDs,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/provider/beatport/client.go

@@ -546,7 +546,7 @@ func (c *Client) normalizeRelease(raw map[string]any, tracks []any) map[string]a
 func (c *Client) normalizeTrack(raw map[string]any) map[string]any {
 	release := mapAny(raw["release"])
 	trackArtistNames := artistNames(raw["artists"])
-	artistName := displayArtistNames(trackArtistNames)
+	artistName := jsonutil.DisplayArtistNames(trackArtistNames)
 	artistID := firstArtistID(raw["artists"])
 	albumArtist := strings.TrimSpace(jsonutil.StringFromAny(release["album_artist_name"]))
 	if albumArtist == "" {
@@ -592,7 +592,7 @@ func (c *Client) normalizeTrackListItem(raw map[string]any) map[string]any {
 		"id":           jsonutil.StringFromAny(raw["id"]),
 		"title":        jsonutil.StringFromAny(raw["name"]),
 		"version":      jsonutil.StringFromAny(raw["mix_name"]),
-		"artist":       map[string]any{"name": displayArtistNames(names)},
+		"artist":       map[string]any{"name": jsonutil.DisplayArtistNames(names)},
 		"album":        map[string]any{"id": jsonutil.NestedString(raw, "release", "id"), "title": jsonutil.NestedString(raw, "release", "name")},
 		"track_number": jsonutil.IntFromAny(raw["number"]),
 	}
@@ -712,32 +712,19 @@ func artistNames(v any) []string {
 	return names
 }
 
-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]
-	}
-}
-
 func (c *Client) releaseArtistName(v any) string {
 	items := sliceAny(v)
 	threshold := c.cfg.Session.Beatport.VariousArtistsThreshold
 	if threshold >= 0 && len(items) > threshold {
 		return "Various Artists"
 	}
-	return displayArtistNames(artistNames(v))
+	return jsonutil.DisplayArtistNames(artistNames(v))
 }
 
 func (c *Client) releaseArtistNameForTracks(releaseArtists any, tracks []any) string {
 	common := commonReleaseArtists(releaseArtists, tracks)
 	if len(common) > 0 {
-		return displayArtistNames(artistNames(common))
+		return jsonutil.DisplayArtistNames(artistNames(common))
 	}
 	return c.releaseArtistName(releaseArtists)
 }