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/library/config_test.go

@@ -0,0 +1,83 @@
+package library
+
+import (
+	"testing"
+)
+
+func TestLibraryConfig_Validate_ValidDirectory(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+
+	modes := []ScanConcurrency{
+		ScanConcurrencyAuto,
+		ScanConcurrencySSD,
+		ScanConcurrencyHDD,
+	}
+
+	for _, mode := range modes {
+		t.Run(string(mode), func(t *testing.T) {
+			t.Parallel()
+
+			c := &Config{
+				DirectoryPath:   Directory(dir),
+				ScanConcurrency: mode,
+			}
+			if err := c.Validate(); err != nil {
+				t.Errorf("Validate() returned unexpected error: %v", err)
+			}
+		})
+	}
+}
+
+func TestLibraryConfig_Validate_NonexistentDirectory(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{
+		DirectoryPath:   "/nonexistent/path/xyz",
+		ScanConcurrency: ScanConcurrencyAuto,
+	}
+
+	err := c.Validate()
+	if err == nil {
+		t.Fatal("Validate() expected error for nonexistent directory, got nil")
+	}
+}
+
+func TestLibraryConfig_Validate_InvalidScanConcurrency(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{
+		DirectoryPath:   Directory(t.TempDir()),
+		ScanConcurrency: "turbo",
+	}
+
+	err := c.Validate()
+	if err == nil {
+		t.Fatal("Validate() expected error for unknown scan concurrency, got nil")
+	}
+}
+
+func TestLibraryConfig_Validate_EmptyDirectory(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{
+		DirectoryPath:   "",
+		ScanConcurrency: ScanConcurrencyAuto,
+	}
+
+	if err := c.Validate(); err != nil {
+		t.Errorf("Validate() returned unexpected error for empty directory: %v", err)
+	}
+}
+
+func TestLibraryConfig_ApplyDefaults(t *testing.T) {
+	t.Parallel()
+
+	c := &Config{}
+	c.ApplyDefaults()
+
+	if c.ScanConcurrency != DefaultScanConcurrency {
+		t.Errorf("ScanConcurrency = %q, want %q", c.ScanConcurrency, DefaultScanConcurrency)
+	}
+}