feat: Add GitHub button to title bar

[plvzfq-rit]

🔗 Linked issue

Resolves #2518

📚 Description

In this pull request, I add a title bar tab which links to the JabRef GitHub repositories.
Some breakpoint changes were also done to the JabRef name (from lg to xl) and to the detection of small screens (from md to lg); this was to fix an issue wherein the title bar appears below the divider when using smaller screens.

It makes navigation to the GitHub repositories easier.

[plvzfq-rit]

Hi! I've added a GitHub logo button to the top right of the page in the title bar. I did not add a search bar for the title bar for now since the site is quite sparse. I also did not add a dark mode button since I was not sure how dark mode for the site should look like. Do let me know how I could improve upon this feature, or if there's something else I should do+

[plvzfq-rit]

By the by, I am not quite sure if I could do this since I'm relatively new to doing open-source contributions, but I had sent parts of code from index.vue to ChatGPT since it was my first time working with Vue and I did not quite understand the code. I did try to delete what I had sent from its memory, as well as chat logs of the exchange. Apologies for this oversight;;;

[plvzfq-rit]

For now, I'll tinker with the code and see how else I could improve the title bar

[tobiasdiez]

Would it be perhaps an idea to start by merging this PR first, and then work on the other improvements in another PR? Just let me know what you prefer!

[plvzfq-rit]

Oh, yeah, we could merge this in first before the other pull request+ I'll have this be ready for review then++

[github-actions]

Deployed cfec8f3 to https://gentle-forest-03418aa03-2560.westeurope.3.azurestaticapps.net

[plvzfq-rit]

Hullo, I've made some changes in this pull request as suggested in #2561; these changes are updated in the first comment in this thread. Do let me know if I can make it better still. For now, as also suggested, I'll have a look into #2176