test: add installed-package smoke test for Shiny app

[Gero1999]

Launches the app against a real R CMD INSTALL (exported-only namespace, build-filtered files) from a temp dir, without load_all. This is the only context that surfaces install-only failures such as bare-name internal calls and resource paths pointing at .Rbuildignore'd files, which the existing load_all-based suite cannot catch.

Hosted in the existing tests job to reuse its Chrome and dependency setup; job pinned to contents: read.

Issue

Closes #

Description

Change description.

Definition of Done

Definition of done, preferably copied from the issue.

How to test

How to test features not covered by unit tests.

Contributor checklist

• Code passes lintr checks
• Code passes all unit tests
• New logic covered by unit tests
• New logic is documented
• App or package changes are reflected in NEWS
• Package version is incremented
• R script works with the new implementation (if applicable)
• Settings upload works with the new implementation (if applicable)
• If any .scss change was done, run data-raw/compile_css.R
• If a package dependency was added/changed, run data-raw/test_suggests_hidden.R

Notes to reviewer

Anything that the reviewer should know before tacking the pull request?

[Gero1999]

As expected here the CI tests failed, which might mean that the test is detecting the issue

[Gero1999]

we need to merge agian the other branch and check