Let a header that paints its own surface show through while sticky

A section in the header set to #ffffff00 for the scrolled page still looked
white: the sticky header element paints its own white surface and a shadow
line underneath, so a transparent section showed that surface instead of the
page, and the line could be neither recoloured nor removed.

When a top-level header section defines a background of its own - normal,
hover or scrolled - the header element now gets `has-own-surface` and paints
nothing. A header without its own design keeps the white surface, so it does
not turn see-through over the content. A text colour alone is not a surface,
and a boxed column is not the whole header.

The check is a Twig function rather than a controller variable because four
paths render the header: page, archive, editor preview and area preview.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

public/assets/site.css

@@ -409,6 +409,10 @@ body.consent-open{padding-bottom:320px}
    Inline geoeffnet stand es sonst in der schmalen Spalte neben der Marke und schob den
    ganzen Kopf auseinander - auf dem Telefon sah es aus, als fehle das Menue. */
 .site-header{position:relative}
+/* Gibt der Kopfbereich seine Flaeche selbst vor, malt der klebende Kopf keine zweite dahinter:
+   sonst zeigte eine durchsichtige Sektion Weiss statt Seite, und die Schattenlinie liesse sich
+   weder faerben noch abschalten. Linie und Schatten kommen dann aus Rahmen und Wirkung. */
+.site-header.is-sticky.has-own-surface{background:transparent;box-shadow:none}
 .site-header .menu-burger[open]>summary{border-radius:8px}
 .site-header .menu-burger-body{position:absolute;left:0;right:0;top:100%;z-index:30;
   padding:15px max(16px,calc((100% - var(--k-container,1180px)) / 2 + 16px)) 25px;