fix: reset focus to app root after unit navigation

[jacobo-dominguez-wgu]

Description

After an internal accessibility audit, the next problem was identified:
When a keyboard or screen-reader user activates the Next or Previous button at the bottom of a unit, focus remains on that button after the new unit loads. The user must manually navigate back up the page to reach the new content, violating WCAG 2.4.3 Focus Order (A).

Solution: After each unit or sequence navigation, programmatically move focus to div.app-container (the MFE root), which is always present regardless of plugin slot customization. If that element is not found, focus falls back to document.body.

Testing

• Unit tests pass
• Manual keyboard navigation test: tab to Next button, press Enter, confirm focus lands on div.app-container and not on the button
• Screen reader test (VoiceOver / NVDA): confirm no unexpected announcement on focus landing at the container

Demo

After

Focus resets to the top after clicking next button:

Screen.Recording.2026-08-24.at.2.12.11.PM.mov

Before

Focus remains on the navigation buttons after clicking next button:

Screen.Recording.2026-08-24.at.2.15.46.PM.mov

AI usage notice

Used Claude Sonnet 4.6 through kiro to assist on this creation of this pr.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.12%. Comparing base (09e57e0) to head (a5bd8a7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2025   +/-   ##
=======================================
  Coverage   93.11%   93.12%           
=======================================
  Files         364      364           
  Lines        5939     5946    +7     
  Branches     1414     1417    +3     
=======================================
+ Hits         5530     5537    +7     
  Misses        391      391           
  Partials       18       18           

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[brian-smith-tcril]

Should navigation stay on the bottom previous/next buttons in this scenario? That feels odd to me when all of the content above it has changed.

I'd like to hear @kblemel's thoughts on this one.

[jacobo-dominguez-wgu]

Should navigation stay on the bottom previous/next buttons in this scenario? That feels odd to me when all of the content above it has changed.

I'd like to hear @kblemel's thoughts on this one.

Before this pr, the focus stayed on the navigation buttons on the bottom even when the content changed, the intention is to move the focus to the content once one of the navigations buttons is clicked.
Or, should we keep the focus on the buttons regardless of the content change?

[brian-smith-tcril]

Should navigation stay on the bottom previous/next buttons in this scenario? That feels odd to me when all of the content above it has changed.
I'd like to hear @kblemel's thoughts on this one.

Before this pr, the focus stayed on the navigation buttons on the bottom even when the content changed, the intention is to move the focus to the content once one of the navigations buttons is clicked. Or, should we keep the focus on the buttons regardless of the content change?

Oh. I mixed up the before and after videos. This PR is making it so the focus goes back to the top upon navigation. That feels correct to me, but I'd still like to hear Kevin's thoughts to make sure there wasn't a specific reason behind the previous behavior I'm unaware of.