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/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
+}