Fix #5867: Clipped admin sidebar text macOS using firefox

[bram-maegerman]

References

• Fixes Admin sidebar icons hide part of the label text #5867

Description

For windows using Firefox, an estimated scrollbar width was already set for the admin-sidebar here. This was not the case yet for macOS.

• Made sure macOS was recognized when setting the class .browser-{browser name}-{OS name}
• Added the appropriate width for macOS
• Added the same width as a default for other OSs, so this shouldn't be an issue anymore in the future.

Instructions for Reviewers

• Open the admin sidebar (make sure to be logged in as an administrator)
• Verify the text is now displayed fully next to the icon, instead of partially hidden by the icon.

Checklist

• My PR is created against the main branch of code (unless it is a backport or is fixing an issue specific to an older branch).
• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• My PR follows all coding best practices based on the Code Conventions Guide
• My PR passes ESLint validation using npm run lint
• My PR doesn't introduce circular dependencies (verified via npm run check-circ-deps)
• My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• My PR aligns with Accessibility guidelines if it makes changes to the user interface.
• My PR uses i18n (internationalization) keys instead of hardcoded English text, to allow for translations.
• My PR includes details on how to test it. I've provided clear instructions to reviewers on how to successfully test this fix or feature.
• If my PR includes new libraries/dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.
• If my PR includes new features or configurations, I've provided basic technical documentation in the PR itself.
• If my PR fixes an issue ticket, I've linked them together.

[tinsch]

I can confirm that these changes fix the original issue 👍 Tested it again on the device where I could reproduce it.
Tested it also in other browsers and on other viewports, looks good to me.