Fix broken file name tabs in mobile

[shujuuu]

Fixes #3157

Changes:
Specifying the React fragment as a div fixed the issue.
Also changed the tab's bg color so it looks like an active tab.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

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[raclim]

Thanks so much for catching this issue and working on this! I realized it might've originated from this PR (https://github.com/processing/p5.js-web-editor/pull/3130/files). It overall seems good to me, so I'm going to merge this in!