[LoneRanger9000]

Summary

Added a Team and a Constitution page, updates site navigation to reflect the new pages, and fixes a bug in the contact form.

New pages

• /team: the current executive plus past executives from 2022 onwards, with LinkedIn links where available.
• /constitution: the society constitutio.
• Both pages share a new header component (src/components/PageHero/).

Navigation

• Added Team and Constitution to the header, the mobile menu and the footer.
• Removed News from the header and mobile menu, since it's no longer maintained. The /articles page itself is kept for now because other pages rely on its stylesheet.

Contact form fix

• If a message fails to send, the form now shows an error with a fallback email address instead of hanging on "Sending....".
• The submit button is disabled while sending, so double clicks don't send duplicate messages.

Docs

• The README explains how to update the team page and the constitution.