Showcase/demo: Tailwind v4 upgrade test setup

[fpbrault]

This PR includes a header for the atomic examples that can be used on both this branch(with Tailwind v4 and the master branch (with Tailwind v3).

This header does a couple things:

• Ensure that the elements in the header and outside of the atomic components are identical so that there is no layout differences.
• Adds a custom theme toggle button to quickly compare the two branches with the default theme and a custom theme.

---

Some changes are known/expected:

• Gradient on horizontal facets in the v4 branch. This gradient was actually missing before due to a bug
  
  

• Added margin under product title on mobile. A bug was preventing this margin from being applied previously.
  
  

• Long product descriptions not cut off anymore in mobile width. This was a bug too!

• Color of line under tabs can be changed with --atomic-neutral.

[github-actions]

Pull Request Report

PR Title

❌ Title should follow the conventional commit spec:
<type>(optional scope): <description>

Example: feat(headless): add result-list controller

Live demo links

• Storybook

• Playwright report

Bundle Size

File	Old (kb)	New (kb)	Change (%)
case-assist	244.2	244.2	0
commerce	355.5	355.5	0
search	415.5	415.5	0
insight	406.7	406.7	0
recommendation	256.3	256.3	0
ssr	409.3	409.3	0
ssr-commerce	373.2	373.2	0