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/theme/config_test.go

@@ -0,0 +1,83 @@
+package theme
+
+import (
+	"testing"
+)
+
+func TestThemeConfig_Validate_ValidValues(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		color string
+		shade BackgroundShade
+	}{
+		{"short hex dark", "#fff", BackgroundDark},
+		{"six-digit hex darker", "#ffd43b", BackgroundDarker},
+		{"black hex light", "#000000", BackgroundLight},
+		{"uppercase hex", "#AABBCC", BackgroundDark},
+		{"mixed case hex", "#aAbBcC", BackgroundDark},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			c := &Config{AccentColor: tt.color, BackgroundShade: tt.shade}
+			if err := c.Validate(); err != nil {
+				t.Errorf("Validate() returned unexpected error: %v", err)
+			}
+		})
+	}
+}
+
+func TestThemeConfig_Validate_InvalidHexColor(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		color string
+	}{
+		{"missing hash", "fff"},
+		{"invalid chars", "#gg0000"},
+		{"wrong length 5", "#12345"},
+		{"word color", "red"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			c := &Config{AccentColor: tt.color, BackgroundShade: BackgroundDark}
+			err := c.Validate()
+			if err == nil {
+				t.Error("Validate() expected error for invalid hex color, got nil")
+			}
+		})
+	}
+}
+
+func TestThemeConfig_Validate_InvalidBackgroundShade(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{AccentColor: "#ffd43b", BackgroundShade: "neon"}
+	err := c.Validate()
+	if err == nil {
+		t.Fatal("Validate() expected error for unknown shade, got nil")
+	}
+}
+
+func TestThemeConfig_ApplyDefaults(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{}
+	c.ApplyDefaults()
+
+	if c.AccentColor != DefaultAccentColor {
+		t.Errorf("AccentColor = %q, want %q", c.AccentColor, DefaultAccentColor)
+	}
+
+	if c.BackgroundShade != DefaultBackgroundShade {
+		t.Errorf("BackgroundShade = %q, want %q", c.BackgroundShade, DefaultBackgroundShade)
+	}
+}