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  (77 lines, superseded by promptSearchSelectionMenu), AddPlaylistByTrackIDs,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/config/config.go

@@ -38,7 +38,6 @@ type ConfigData struct {
 	Yandex       YandexConfig                 `toml:"yandex"`
 	Beatport     BeatportConfig               `toml:"beatport"`
 	Soundcloud   SoundcloudConfig             `toml:"soundcloud"`
-	Youtube      YoutubeConfig                `toml:"youtube"`
 	Database     DatabaseConfig               `toml:"database"`
 	Conversion   ConversionConfig             `toml:"conversion"`
 	QobuzFilters QobuzDiscographyFilterConfig `toml:"qobuz_filters"`
@@ -61,13 +60,12 @@ type DownloadsConfig struct {
 }
 
 type QobuzConfig struct {
-	Quality          int      `toml:"quality"`
-	DownloadBooklets bool     `toml:"download_booklets"`
-	UseAuthToken     bool     `toml:"use_auth_token"`
-	EmailOrUserID    string   `toml:"email_or_userid"`
-	PasswordOrToken  string   `toml:"password_or_token"`
-	AppID            string   `toml:"app_id"`
-	Secrets          []string `toml:"secrets"`
+	Quality         int      `toml:"quality"`
+	UseAuthToken    bool     `toml:"use_auth_token"`
+	EmailOrUserID   string   `toml:"email_or_userid"`
+	PasswordOrToken string   `toml:"password_or_token"`
+	AppID           string   `toml:"app_id"`
+	Secrets         []string `toml:"secrets"`
 }
 
 type TidalConfig struct {
@@ -112,12 +110,6 @@ type SoundcloudConfig struct {
 	AppVersion string `toml:"app_version"`
 }
 
-type YoutubeConfig struct {
-	Quality              int    `toml:"quality"`
-	DownloadVideos       bool   `toml:"download_videos"`
-	VideoDownloadsFolder string `toml:"video_downloads_folder"`
-}
-
 type DatabaseConfig struct {
 	DownloadsEnabled       bool   `toml:"downloads_enabled"`
 	DownloadsPath          string `toml:"downloads_path"`
@@ -177,8 +169,7 @@ type CLIConfig struct {
 }
 
 type MiscConfig struct {
-	Version         string `toml:"version"`
-	CheckForUpdates bool   `toml:"check_for_updates"`
+	Version string `toml:"version"`
 }
 
 func Load(path string) (*Config, error) {
@@ -366,9 +357,8 @@ func DefaultConfigData() ConfigData {
 			VerifySSL:            true,
 		},
 		Qobuz: QobuzConfig{
-			Quality:          3,
-			DownloadBooklets: true,
-			UseAuthToken:     false,
+			Quality:      3,
+			UseAuthToken: false,
 		},
 		Tidal: TidalConfig{
 			Quality:        3,
@@ -389,11 +379,6 @@ func DefaultConfigData() ConfigData {
 		Soundcloud: SoundcloudConfig{
 			Quality: 0,
 		},
-		Youtube: YoutubeConfig{
-			Quality:              0,
-			DownloadVideos:       false,
-			VideoDownloadsFolder: filepath.Join(downloadsFolder, "YouTubeVideos"),
-		},
 		Database: DatabaseConfig{
 			DownloadsEnabled:       true,
 			DownloadsPath:          filepath.Join(appDir, "downloads.db"),
@@ -437,8 +422,7 @@ func DefaultConfigData() ConfigData {
 			MaxSearchResults: 100,
 		},
 		Misc: MiscConfig{
-			Version:         CurrentConfigVersion,
-			CheckForUpdates: true,
+			Version: CurrentConfigVersion,
 		},
 	}
 
@@ -498,9 +482,6 @@ func applyRuntimeDefaults(data *ConfigData) {
 	if data.Database.FailedDownloadsPath == "" {
 		data.Database.FailedDownloadsPath = filepath.Join(appDir, "failed_downloads.db")
 	}
-	if data.Youtube.VideoDownloadsFolder == "" {
-		data.Youtube.VideoDownloadsFolder = filepath.Join(data.Downloads.Folder, "YouTubeVideos")
-	}
 }
 
 // CloneConfigData returns a deep copy of in. The slice fields must not be