feat(nav): add mobile nav toggle

[Don-Damiano]

Motivation

• Improve mobile navigation by adding a toggleable menu and make the nav keyboard-accessible and ARIA-friendly.

Description

• Add a hamburger-style toggle button (button.nav-toggle) with aria-controls and aria-expanded and wrap links in div.navlinks with id="primary-nav" and data-open state in src/components/Header.astro.
• Include an inline script that updates aria-expanded and the data-open attribute on click to drive the visual state.
• Update src/styles/global.css with .nav-header, .nav-toggle, :focus-visible styles, and a @media (max-width: 768px) rule that switches .nav to a column layout, hides .navlinks by default, and shows it when data-open="true".

Testing

• Launched the dev server with npm run dev and the Astro server started successfully (served at http://localhost:4321/).
• Captured a mobile viewport screenshot via a Playwright script to verify the collapsed menu and toggle behavior, which completed successfully and produced an artifact.

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Codex Task