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  (77 lines, superseded by promptSearchSelectionMenu), AddPlaylistByTrackIDs,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

@@ -0,0 +1,184 @@
+package jsonutil
+
+import "testing"
+
+func TestStringFromAny(t *testing.T) {
+	tests := []struct {
+		name string
+		in   any
+		want string
+	}{
+		{"string", "hi", "hi"},
+		{"int", 7, "7"},
+		{"int64", int64(8), "8"},
+		{"float whole", float64(9), "9"},
+		{"float fractional", 44.1, "44.1"},
+		{"float no trailing zeroes", float64(100), "100"},
+		{"nil", nil, ""},
+		{"bool unsupported", true, ""},
+		{"map unsupported", map[string]any{}, ""},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := StringFromAny(tc.in); got != tc.want {
+				t.Fatalf("StringFromAny(%#v) = %q, want %q", tc.in, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestIntFromAny(t *testing.T) {
+	tests := []struct {
+		in   any
+		want int
+	}{
+		{5, 5},
+		{int32(6), 6},
+		{int64(7), 7},
+		{8.9, 8},
+		{"10", 10},
+		{"  11  ", 11},
+		{"not a number", 0},
+		{nil, 0},
+		{true, 0},
+	}
+	for _, tc := range tests {
+		if got := IntFromAny(tc.in); got != tc.want {
+			t.Errorf("IntFromAny(%#v) = %d, want %d", tc.in, got, tc.want)
+		}
+	}
+}
+
+func TestFloatFromAny(t *testing.T) {
+	tests := []struct {
+		in   any
+		want float64
+	}{
+		{1.5, 1.5},
+		{2, 2},
+		{int64(3), 3},
+		{"4.5", 0}, // strings are deliberately not parsed here
+		{nil, 0},
+	}
+	for _, tc := range tests {
+		if got := FloatFromAny(tc.in); got != tc.want {
+			t.Errorf("FloatFromAny(%#v) = %v, want %v", tc.in, got, tc.want)
+		}
+	}
+}
+
+func TestBoolFromAny(t *testing.T) {
+	tests := []struct {
+		in   any
+		want bool
+	}{
+		{true, true},
+		{false, false},
+		{"true", true},
+		{"TRUE", true},
+		{" yes ", true},
+		{"1", true},
+		{"0", false},
+		{"no", false},
+		{1, true},
+		{0, false},
+		{int64(2), true},
+		{0.0, false},
+		{1.5, true},
+		{nil, false},
+	}
+	for _, tc := range tests {
+		if got := BoolFromAny(tc.in); got != tc.want {
+			t.Errorf("BoolFromAny(%#v) = %v, want %v", tc.in, got, tc.want)
+		}
+	}
+}
+
+func TestFirstNonEmpty(t *testing.T) {
+	if got := FirstNonEmpty("", "  ", "found", "later"); got != "found" {
+		t.Errorf("got %q", got)
+	}
+	if got := FirstNonEmpty("  trimmed  "); got != "trimmed" {
+		t.Errorf("got %q, want trimmed", got)
+	}
+	if got := FirstNonEmpty("", "   "); got != "" {
+		t.Errorf("got %q, want empty", got)
+	}
+	if got := FirstNonEmpty(); got != "" {
+		t.Errorf("got %q, want empty", got)
+	}
+}
+
+func TestNestedMap(t *testing.T) {
+	m := map[string]any{"album": map[string]any{"title": "X"}, "n": 1}
+	if got := NestedMap(m, "album")["title"]; got != "X" {
+		t.Errorf("got %v", got)
+	}
+	if got := NestedMap(m, "missing"); len(got) != 0 {
+		t.Errorf("missing key should give empty map, got %v", got)
+	}
+	if got := NestedMap(m, "n"); len(got) != 0 {
+		t.Errorf("non-map value should give empty map, got %v", got)
+	}
+}
+
+func TestNestedAnyAndString(t *testing.T) {
+	m := map[string]any{
+		"album": map[string]any{
+			"artist": map[string]any{"name": "Nina Simone", "id": float64(42)},
+		},
+	}
+	if got := NestedString(m, "album", "artist", "name"); got != "Nina Simone" {
+		t.Errorf("got %q", got)
+	}
+	if got := NestedString(m, "album", "artist", "id"); got != "42" {
+		t.Errorf("numeric id = %q, want 42", got)
+	}
+	if got := NestedString(m, "album", "nope", "name"); got != "" {
+		t.Errorf("missing path = %q, want empty", got)
+	}
+	if got := NestedAny(m, "album", "artist", "name"); got != "Nina Simone" {
+		t.Errorf("got %v", got)
+	}
+	if got := NestedAny(m); got == nil {
+		t.Error("no keys should return the map itself")
+	}
+	// Traversing through a non-map must not panic.
+	if got := NestedAny(m, "album", "artist", "name", "deeper"); got != nil {
+		t.Errorf("over-deep path = %v, want nil", got)
+	}
+}
+
+func TestTitleCase(t *testing.T) {
+	tests := []struct{ in, want string }{
+		{"qobuz", "Qobuz"},
+		{"Tidal", "Tidal"},
+		{"", ""},
+		{"1st", "1st"},
+		{"ä", "ä"}, // only ASCII is uppercased
+	}
+	for _, tc := range tests {
+		if got := TitleCase(tc.in); got != tc.want {
+			t.Errorf("TitleCase(%q) = %q, want %q", tc.in, got, tc.want)
+		}
+	}
+}
+
+func TestDisplayArtistNames(t *testing.T) {
+	tests := []struct {
+		in   []string
+		want string
+	}{
+		{nil, ""},
+		{[]string{}, ""},
+		{[]string{"A"}, "A"},
+		{[]string{"A", "B"}, "A & B"},
+		{[]string{"A", "B", "C"}, "A, B & C"},
+		{[]string{"A", "B", "C", "D"}, "A, B, C & D"},
+	}
+	for _, tc := range tests {
+		if got := DisplayArtistNames(tc.in); got != tc.want {
+			t.Errorf("DisplayArtistNames(%v) = %q, want %q", tc.in, got, tc.want)
+		}
+	}
+}