Use spec font stack

[weeman1337]

🤷

Left: spec font stack as suggested here
Right: current

Replaces #2355

✔️ Checklist

• Check for common mistakes:
  • Wrap plain URLs in <> to linkify them (learn more).
  • Use the right level of headings: The page title will use a level 1 headings, so your headings should use level 2 and below.
  • Use internal links: when linking to another page on https://matrix.org, use the Zola [label](@/target.md) syntax.
• For blog posts:
  • Verify the date and post ordering on the /blog page, especially for multiple posts on the same day. Prefer UTC format, e.g. 2025-12-01T14:00:00Z for Dec 1st, 2025, 2pm UTC.
  • Set the correct author and category. Browse existing ones at https://matrix.org/author/ and https://matrix.org/category/ to match them.
• Let us know if you are contributing in a specific role, such as on behalf of an organisation or team, for example.
• Let us know if your PR is time-sensitive in any way.
• Mention any issues related to the PR. Use GitHub keywords as appropriate.
• Your individual commits or pull request is signed off.

[HarHarLinks]

Nimbus Sans (Helvetica/Neue fallback)	Inter
Liberation Sans (Arial fallback)	Roboto

weeman's Inter seems to look quite different from mine.

I think which ever way we go, we should probably ship a font to ensure consistency; consistency is the main problem I see with the current Helvetica implementation.

[HarHarLinks]

If we decide to ship Inter, we probably want to incorporate most of what was touched in this spec PR https://github.com/matrix-org/matrix-spec/pull/2000/files, incl the script and perhaps keeping a bunch of the international formats since the blog may contain some more scripts than the spec.