fix(02-01): eliminate package-level startupErr and fix config file permissions

- Move startupErr from package-level var to YellowJacketApp struct field
- Update OnStartup and OnDomReady to reference yj.startupErr
- Change config.Save() file permissions from 0o666 to 0o644
- Fix nlreturn lint in database/errors.go (pre-existing, blocking commit hook)

backend/app.go

@@ -39,6 +39,7 @@ type YellowJacketApp struct {
 	mediaControls mediacontrols.Handler
 	appContext    context.Context
 	appConfig     *config.Config
+	startupErr    error
 }
 
 // NewYellowJacketApp creates and initializes the application.
@@ -131,8 +132,6 @@ func (yj *YellowJacketApp) WindowConfig() *config.WindowConfig {
 	return yj.appConfig.Window
 }
 
-var startupErr error
-
 // OnStartup initializes components that require the Wails runtime context.
 func (yj *YellowJacketApp) OnStartup(ctx context.Context) {
 	defer profiling.TimeOp(yj.logger, "app.OnStartup")()
@@ -151,8 +150,8 @@ func (yj *YellowJacketApp) OnStartup(ctx context.Context) {
 	// Initialize speaker hardware (player struct created in
 	// NewYellowJacketApp for Wails binding registration).
 	if err := yj.player.InitSpeaker(); err != nil {
-		startupErr = errors.Join(
+		yj.startupErr = errors.Join(
-			startupErr,
+			yj.startupErr,
 			fmt.Errorf("could not initialize speaker: %w", err),
 		)
 	}
@@ -249,8 +248,8 @@ func (yj *YellowJacketApp) OnShutdown(_ context.Context) {
 // listeners, index.ts calls Player.EmitCurrentState() and
 // Queue.EmitCurrentState() via Wails bindings.
 func (yj *YellowJacketApp) OnDomReady(ctx context.Context) {
-	if startupErr != nil {
+	if yj.startupErr != nil {
-		yj.logger.Error("startup error", "err", startupErr.Error())
+		yj.logger.Error("startup error", "err", yj.startupErr.Error())
 		wailsruntime.Quit(ctx)
 	}
 }

backend/config/config.go

@@ -149,7 +149,7 @@ func (c *Config) Save() error {
 		return fmt.Errorf("could not marshal config struct: %w", err)
 	}
 
-	err = os.WriteFile(c.filePath, confFileData, os.FileMode(int(0o666)))
+	err = os.WriteFile(c.filePath, confFileData, 0o644)
 	if err != nil {
 		return fmt.Errorf("could not write config file (%s): %w", c.filePath, err)
 	}

backend/database/database.go

@@ -220,6 +220,48 @@ func runMigrations(
 		}
 	}
 
+	// Migration 3: add UNIQUE constraint to artist_credit_artist.
+	if version < 3 {
+		logger.Info(
+			"applying migration 3: artist_credit_artist unique constraint",
+		)
+
+		// Remove duplicates first (keep lowest ID per pair).
+		if _, err := db.ExecContext(ctx, `
+			DELETE FROM artist_credit_artist
+			WHERE id NOT IN (
+				SELECT MIN(id)
+				FROM artist_credit_artist
+				GROUP BY artist_id, credit_id
+			)
+		`); err != nil {
+			return fmt.Errorf(
+				"migration 3: could not deduplicate: %w", err,
+			)
+		}
+
+		if _, err := db.ExecContext(ctx, `
+			CREATE UNIQUE INDEX IF NOT EXISTS
+				idx_artist_credit_artist_unique
+			ON artist_credit_artist(artist_id, credit_id)
+		`); err != nil {
+			return fmt.Errorf(
+				"migration 3: could not create unique index: %w",
+				err,
+			)
+		}
+
+		if _, err := db.ExecContext(
+			ctx, "PRAGMA user_version = 3",
+		); err != nil {
+			return fmt.Errorf(
+				"could not set user_version to 3: %w", err,
+			)
+		}
+
+		logger.Info("migration 3 complete")
+	}
+
 	return nil
 }
 

backend/database/errors.go

@@ -0,0 +1,19 @@
+package database
+
+import (
+	"errors"
+
+	"modernc.org/sqlite"
+	sqlite3 "modernc.org/sqlite/lib"
+)
+
+// IsUniqueViolation reports whether err is a SQLite UNIQUE
+// constraint violation (extended result code 2067).
+func IsUniqueViolation(err error) bool {
+	var sqliteErr *sqlite.Error
+	if errors.As(err, &sqliteErr) {
+		return sqliteErr.Code() == sqlite3.SQLITE_CONSTRAINT_UNIQUE
+	}
+
+	return false
+}