Fix accessibility and robustness findings from the branch review (develop)

[mlaetitia]

Related to #

Proposed changes

• Escape closes a mega menu panel from inside it, and closes one opened by hover.
• Green text and the mobile CTA use the darker brand preset so they meet WCAG AA contrast.
• Placeholder submenu links are only click-blocked on desktop, so mobile accordion rows respond to taps.
• The mobile caret is centred on its row.
• The featured card's link darkens on hover instead of lightening.
• aria-haspopup removed from the panel triggers.
• The nav_menu_link_attributes callback no longer declares typed parameters.

Why are these changes being made?

Findings from a review of the branch, each reproduced before the fix and re-checked after.

• Escape only closed a panel when focus was already on the trigger, and never closed one opened by hover.
• Green text and the mobile CTA failed AA contrast at 3.13:1 on white.
• Tapping a mobile row's label did not expand it; only the caret worked.
• The mobile caret sat 5px below the centre of its row.
• Hovering the featured card's link made it lighter than its resting colour instead of darker.
• aria-haspopup="true" promised screen reader users arrow-key menu navigation this component does not implement.
• The nav_menu_link_attributes callback declared typed parameters, which would fatal any page rendering a menu through a plugin's walker.

Testing instructions

Above 1200px:

• Tab into a dropdown item, move into the panel, then press Escape from a link inside it. The panel should close and focus should return to the top-level item.
• Hover a dropdown open, do not move the pointer, press Escape. The panel and the dimmed background should both go. Moving away and back should reopen it normally.
• The eyebrow, the featured card's heading and its link should read as a deeper green. Hovering the link should darken it further and underline it.
• Panels should still open on hover and on keyboard focus, and columns should still fill top to bottom before wrapping.

Below 1200px:

• Tap a top-level row on the label rather than the caret. It should expand.
• The caret should sit level with the row's text, both collapsed and expanded.
• The Get Involved button should be a deeper green with white text.

Screen reader, if convenient: top-level items should announce as collapsed or expanded, and no longer as a menu popup.