Summary

Fixes #435 — The navigation hamburger menu button and nav element lacked ARIA attributes, making the site inaccessible for screen reader users.

Changes made (src/components/header/Header.js only)

• Added aria-label="Toggle navigation menu" and aria-controls="nav-menu" to the hamburger checkbox input — screen readers now announce the button's purpose and its controlled region
• Added aria-hidden="true" to the menu label — hides the decorative icon from assistive tech since the checkbox is already announced
• Added id="nav-menu", role="navigation", and aria-label="Main navigation" to the <ul> — makes it a discoverable landmark and satisfies the aria-controls reference

What was NOT changed

• No styling, layout, or behaviour modified
• No other files touched

Test plan

• 6 unit tests written and passing (Header.test.js)
  • renders without crashing
  • hamburger checkbox has aria-label for toggle
  • hamburger checkbox has aria-controls pointing to nav menu
  • menu label is hidden from screen readers via aria-hidden
  • nav ul has correct id, role, and aria-label
  • aria-controls on checkbox matches id of nav ul
• Existing functionality intact — no visual or behavioural regressions

Closes #435