feat: Upgrade to Next.js 15 and enable Turbopack

This commit applies the following changes:

1.  **Upgrades Next.js and related dependencies in `apps/web/package.json`:**
    *   `next` updated to `^15.0.0`
    *   `react` and `react-dom` updated to `^18.3.1`
    *   `eslint-config-next` updated to `^15.1.1`

2.  **Enables Turbopack for development:**
    *   The `dev` script in `apps/web/package.json` is now `next dev --turbo`.

3.  **Updates `apps/web/next.config.js` for Next.js 15 compatibility:**
    *   `experimental.serverComponentsExternalPackages` has been changed to the stable `serverExternalPackages`.

These changes are part of the initiative to upgrade to Next.js 15 and leverage Turbopack for faster development builds.

Author: google-labs-jules[bot]