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  (77 lines, superseded by promptSearchSelectionMenu), AddPlaylistByTrackIDs,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

@@ -269,14 +269,6 @@ func (c *Client) GetMetadata(ctx context.Context, item, mediaType string) (map[s
 	return resp, nil
 }
 
-func (c *Client) GetTrackMetadata(ctx context.Context, id string) (*TrackMetadata, error) {
-	raw, err := c.GetMetadata(ctx, id, "track")
-	if err != nil {
-		return nil, err
-	}
-	return ParseTrackMetadata(raw)
-}
-
 func (c *Client) Search(ctx context.Context, mediaType, query string, limit int) ([]map[string]any, error) {
 	if !c.LoggedIn() {
 		return nil, errNotLoggedIn