z-index set by Quarto breaks modals when `page-layout: full`

Bug description

Quarto sets various SASS variables to redefine z-index of several Bootstrap components

• https://github.com/quarto-dev/quarto-cli/blob/c1c5a4308f77fa2674082f41be12446cdcf415fd/src/resources/formats/html/bootstrap/dist/scss/_variables.scss#L1141-L1151

But either something is wrongly set or is not set for when page-layout: full.

Steps to reproduce

---
title: "Reproducible Quarto Document"
format: html
page-layout: full
engine: markdown
---

Here is a modal:

```{=html}
<!-- Button trigger modal -->
<button type="button" class="btn btn-primary" data-bs-toggle="modal" data-bs-target="#exampleModal">
  Launch demo modal
</button>

<!-- Modal -->
<div class="modal fade" id="exampleModal" tabindex="-1" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <h1 class="modal-title fs-5" id="exampleModalLabel">Modal title</h1>
        <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>
```

{{< lipsum 1 >}}

Actual behavior

https://github.com/user-attachments/assets/1248829b-6f70-40c7-a0c4-1711516f2b6d

Expected behavior

https://github.com/user-attachments/assets/35e64eb3-2b47-4016-9fb2-04d84cbdee4c

Your environment

No response

Quarto check output

Quarto 99.9.9
[✓] Checking environment information...
      Quarto cache location: /Users/mcanouil/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.4.0: OK
      Dart Sass version 1.85.1: OK
      Deno version 1.46.3: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      commit: c1c5a4308f77fa2674082f41be12446cdcf415fd
      Path: /Users/mcanouil/Projects/quarto/quarto-cli/package/dist/bin

[✓] Checking tools....................OK
      TinyTeX: v2025.03
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/mcanouil/Library/TinyTeX/bin/universal-darwin
      Version: 2024

[✓] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: /Applications/Brave Browser.app/Contents/MacOS/Brave Browser
      Source: QUARTO_CHROMIUM

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.13.1
      Path: /Users/mcanouil/Projects/quarto/quarto-playground/.venv/bin/python3
      Jupyter: 5.7.2
      Kernels: julia-1.11, python3

[✓] Checking Jupyter engine render....OK

(/) Checking R installation...........ℹ R version 4.4.3 (2025-02-28)
! Config '~/.Rprofile' was loaded!
[✓] Checking R installation...........OK
      Version: 4.4.3
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Users/mcanouil/Projects/quarto/quarto-playground/renv/library/macos/R-4.4/aarch64-apple-darwin20
        - /Users/mcanouil/Library/Caches/org.R-project.R/R/renv/sandbox/macos/R-4.4/aarch64-apple-darwin20/f7156815
      knitr: 1.49
      rmarkdown: 2.29

[✓] Checking Knitr engine render......OK