[A11y] Add proper disclosure semantics to desktop nav dropdown

[andrewy1n]

Problem

Desktop navigation dropdown toggles in src/components/NavDropdown.tsx/src/components/Header.tsx do not expose disclosure state with aria-expanded and do not associate trigger/menu via aria-controls.

This makes dropdown state unclear for assistive technologies and reduces keyboard/screen-reader usability.

Scope

• Add disclosure semantics to desktop dropdown trigger(s): aria-expanded, aria-controls, stable id relationships.
• Ensure keyboard interactions are robust (open/close via Enter/Space, Escape to close, focus returns to trigger when closed).
• Confirm dropdown content has appropriate semantics/labels.

Acceptance Criteria

• Trigger announces open/closed state to screen readers.
• Trigger and menu are linked via IDs.
• Escape key closes open dropdown reliably.
• Keyboard navigation is test-covered.

Files

• src/components/NavDropdown.tsx
• src/components/Header.tsx
• e2e/header-mobile-menu.spec.ts (extend or add desktop-specific spec)