Revert "Use fileURLToPath method"

[gentamura]

The regression stems from PR #7 (commit 740a5c0), which changed schemaPath to:

export const schemaPath = fileURLToPath(
  new URL("./schema/index.js", import.meta.url),
);

That line triggers the runtime error under Next.js. To fix it, revert that change and replace it with the safe version:

const schemaUrl = new URL("./schema/index.js", import.meta.url);
export const schemaPath = fileURLToPath(schemaUrl.href);

This ensures fileURLToPath receives a plain string, avoiding the cross‑realm URL issue.