Bump astro from 4.16.18 to 5.15.9 in /app/app

Bumps [astro](https://github.com/withastro/astro/tree/HEAD/packages/astro) from 4.16.18 to 5.15.9.
- [Release notes](https://github.com/withastro/astro/releases)
- [Changelog](https://github.com/withastro/astro/blob/main/packages/astro/CHANGELOG.md)
- [Commits](https://github.com/withastro/astro/commits/astro@5.15.9/packages/astro)

---
updated-dependencies:
- dependency-name: astro
  dependency-version: 5.15.9
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]