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  (77 lines, superseded by promptSearchSelectionMenu), AddPlaylistByTrackIDs,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_test.go

@@ -208,7 +208,7 @@ func TestGetArtistPagination(t *testing.T) {
 
 func newTestClient(t *testing.T) *Client {
 	t.Helper()
-	d := config.DefaultConfigData()
+	d := testConfigData()
 	d.Qobuz.AppID = "12345"
 	cfg := &config.Config{File: d, Session: d}
 	return New(cfg)
@@ -248,7 +248,7 @@ func TestLoginRefreshesAppCredentialsWhenSecretInvalid(t *testing.T) {
 	}))
 	defer ts.Close()
 
-	d := config.DefaultConfigData()
+	d := testConfigData()
 	d.Qobuz.EmailOrUserID = "user@example.com"
 	d.Qobuz.PasswordOrToken = "hash"
 	d.Qobuz.AppID = "old-app"
@@ -302,7 +302,7 @@ func TestLoginRetriesAfterRefreshingAppCredentials(t *testing.T) {
 	}))
 	defer ts.Close()
 
-	d := config.DefaultConfigData()
+	d := testConfigData()
 	d.Qobuz.EmailOrUserID = "user@example.com"
 	d.Qobuz.PasswordOrToken = "hash"
 	d.Qobuz.AppID = "old-app"
@@ -396,7 +396,7 @@ func qobuzSecretSig(requestTS, secret string) string {
 }
 
 func TestRefreshAppCredentialsRejectsEmptyData(t *testing.T) {
-	d := config.DefaultConfigData()
+	d := testConfigData()
 	c := New(&config.Config{File: d, Session: d})
 	c.fetchCfg = func(context.Context) (string, []string, error) {
 		return "", []string{" "}, nil
@@ -406,3 +406,13 @@ func TestRefreshAppCredentialsRejectsEmptyData(t *testing.T) {
 		t.Fatalf("expected error for empty refreshed app credentials")
 	}
 }
+
+// testConfigData returns defaults with request throttling disabled.
+//
+// At the default 60 requests/minute the rate limiter sleeps 1s per request,
+// which accounted for most of this suite's runtime (deezer alone was 31s).
+func testConfigData() config.ConfigData {
+	d := config.DefaultConfigData()
+	d.Downloads.RequestsPerMinute = -1
+	return d
+}