Keep the header at full height, and make its white background span the page

[mlaetitia]

Related to #

Proposed changes

• Keep the header at full height on scroll. Only the logo shrinks, so nothing below the header moves.
• Make the white background span the full page width whenever the header is white, scrolled or with a megamenu open.
• Centre the logo and nav in the scrolled header, which an unbalanced top padding was pushing low.

Why are these changes being made?

• The header is sticky but still sits in flow, so shrinking it lifted the page and the nav appeared to jump on a small scroll.
• The design asks for a full-width white background. The theme insets the page wrapper by 5vw above 1200px, and the background was painted on the header box, so it stopped short of both edges. It now sits on a layer that escapes that inset, the same way the megamenu panel already did.

Testing instructions

• Above 1200px, scroll down: the white background reaches both edges, the header keeps its height, and only the logo shrinks.
• Hover a megamenu item, scrolled and unscrolled: white veil and page dim as before, with its bottom border.
• Check an AI template page scrolled: the green trim spans the page with the white.
• Check a dropdown while scrolled: it opens instead of being clipped.

🤖 Generated with Claude Code

https://claude.ai/code/session_011rCHPCdbV5C5dDJjEmMDuy