Manager: Capitalize GitHub and open the contributors link in a new tab on mobile

[valentinpalkovic]

What I did

The mobile About page labeled its GitHub link "Github".
It also opened the "Storybook Community" contributors link in the current tab, while every other external link on the page opens in a new one.

This PR fixes both, and gives the Default story a play function that checks the four links by their accessible names.
The AboutFocusTrapped story in MobileNavigation.stories.tsx now looks for the link by its new name.

These changes were split out of #36137, where they did not belong.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

1. Run cd code && yarn storybook:ui.
2. Open the Mobile/About Default story.
3. Confirm the first link reads "GitHub".
4. Click "Storybook Community" and confirm it opens in a new tab.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Essentials

Run ID: 1747a3df-db74-4277-808f-a305f2a16437

📥 Commits

Reviewing files that changed from the base of the PR and between 16efdfc and 80df686.

📒 Files selected for processing (3)

• code/core/src/manager/components/mobile/about/MobileAbout.stories.tsx
• code/core/src/manager/components/mobile/about/MobileAbout.tsx
• code/core/src/manager/components/mobile/navigation/MobileNavigation.stories.tsx

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Walkthrough

The mobile About view corrects the GitHub label and opens the Storybook Community link in a new tab. Storybook stories check the visibility of About links and the updated GitHub accessible name.

Changes

Mobile About links

Layer / File(s)	Summary
Update mobile About links and checks code/core/src/manager/components/mobile/about/MobileAbout.tsx, code/core/src/manager/components/mobile/about/MobileAbout.stories.tsx, code/core/src/manager/components/mobile/navigation/MobileNavigation.stories.tsx	The GitHub label changes to “GitHub.” The Storybook Community link opens in a new tab and retains rel="noopener noreferrer". The About story checks that the GitHub, Documentation, Chromatic, and Storybook Community links are visible. The navigation story expects the updated GitHub accessible name.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to 80df6

The About links and their accessible names are consistent, and the Community link opens in a new tab safely. No material merge risk is evident.

✨ Finishing Touches

📝 Generate docstrings

• Commit to this branch
• Create a new PR

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