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/favorites/config_test.go

@@ -0,0 +1,49 @@
+package favorites
+
+import (
+	"testing"
+)
+
+func TestFavoritesConfig_Validate_ValidIconStyles(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		style IconStyle
+	}{
+		{"heart", IconHeart},
+		{"star", IconStar},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			c := &Config{IconStyle: tt.style}
+			if err := c.Validate(); err != nil {
+				t.Errorf("Validate() returned unexpected error: %v", err)
+			}
+		})
+	}
+}
+
+func TestFavoritesConfig_Validate_InvalidIconStyle(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{IconStyle: "diamond"}
+	err := c.Validate()
+	if err == nil {
+		t.Fatal("Validate() expected error for unknown icon style, got nil")
+	}
+}
+
+func TestFavoritesConfig_ApplyDefaults(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{}
+	c.ApplyDefaults()
+
+	if c.IconStyle != DefaultIconStyle {
+		t.Errorf("IconStyle = %q, want %q", c.IconStyle, DefaultIconStyle)
+	}
+}