fix: Write frontend build files into build dir when it is outside project dir

[ingokegel]

Description

When the build dir is relocated outside the project dir, vaadinPrepareFrontend wrote vaadin-dev-server-settings.json and the dev-bundle output paths under the source tree. buildFolder() is now always relative to projectDir.

Fixes #24804

Type of change

• [ x] Bugfix
• Feature

Checklist

• [x ] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [x ] I have added a description following the guideline.
• [ x] The issue is created in the corresponding repository and I have referenced it.
• [ x] I have added tests to ensure my change is effective and works as intended.
• [ x] New and existing tests are passing locally with my change.
• [ x] I have performed self-review and corrected misspellings.