test(04-02): add config and sub-config validation tests

- Theme: hex color regex + background shade enum validation
- Tracklist: column ID recognition + duplicate detection
- Favorites: icon style enum validation
- Library: directory existence + scan concurrency mode validation
- Config: load/save roundtrip, missing file handling, composed errors, defaults

backend/tracklist/config_test.go

@@ -0,0 +1,73 @@
+package tracklist
+
+import (
+	"testing"
+)
+
+func TestTrackListConfig_Validate_ValidColumns(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{
+		Columns: []Column{
+			{ID: ColTrackName},
+			{ID: ColArtistName},
+			{ID: ColAlbum},
+			{ID: ColTrackLength},
+			{ID: ColGenre},
+		},
+	}
+
+	if err := c.Validate(); err != nil {
+		t.Errorf("Validate() returned unexpected error: %v", err)
+	}
+}
+
+func TestTrackListConfig_Validate_UnknownColumnID(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{
+		Columns: []Column{
+			{ID: ColTrackName},
+			{ID: "nonexistent"},
+		},
+	}
+
+	err := c.Validate()
+	if err == nil {
+		t.Fatal("Validate() expected error for unknown column ID, got nil")
+	}
+}
+
+func TestTrackListConfig_Validate_DuplicateColumn(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{
+		Columns: []Column{
+			{ID: ColTrackName},
+			{ID: ColArtistName},
+			{ID: ColTrackName},
+		},
+	}
+
+	err := c.Validate()
+	if err == nil {
+		t.Fatal("Validate() expected error for duplicate column ID, got nil")
+	}
+}
+
+func TestTrackListConfig_ApplyDefaults(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{}
+	c.ApplyDefaults()
+
+	if len(c.Columns) != len(DefaultColumns) {
+		t.Fatalf("Columns length = %d, want %d", len(c.Columns), len(DefaultColumns))
+	}
+
+	for i, col := range c.Columns {
+		if col.ID != DefaultColumns[i].ID {
+			t.Errorf("Columns[%d].ID = %q, want %q", i, col.ID, DefaultColumns[i].ID)
+		}
+	}
+}