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  (77 lines, superseded by promptSearchSelectionMenu), AddPlaylistByTrackIDs,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>

config.toml.example

@@ -21,9 +21,6 @@ verify_ssl = true
 # Quality ladder (global scale is 0-4, Qobuz supports 1-4):
 # 1 = MP3 320, 2 = FLAC 16/44.1, 3 = FLAC 24-bit <=96kHz, 4 = FLAC 24-bit <=192kHz
 quality = 3
-# Download booklet PDFs when available
-download_booklets = true
-
 # Authenticate using Qobuz auth token instead of email/password hash
 use_auth_token = false
 # If use_auth_token=true, set your user id. Otherwise set your email.
@@ -101,14 +98,6 @@ quality = 0
 client_id = ""
 app_version = ""
 
-[youtube]
-# Only 0 is currently supported
-quality = 0
-# Download video streams together with audio when supported
-download_videos = false
-# Folder used for video outputs
-video_downloads_folder = "/path/to/StreamripDownloads/YouTubeVideos"
-
 [database]
 # Track IDs already downloaded are stored here and skipped next time
 downloads_enabled = true
@@ -189,5 +178,3 @@ max_search_results = 100
 [misc]
 # Metadata used for config compatibility checks
 version = "2.2.0"
-# Notify when a new version is available
-check_for_updates = true