Make header divider a single continuous line like Handsontable docs

The toolbar's horizontal divider was constructed from `border-top` on
each `.hf-nav__link` element, so it only rendered where the framed nav
tabs sat. To the left of the first tab and to the right of the Support
dropdown, the line broke off and left visible gaps.

Mirror Handsontable's `.header-row-2::before` pattern: add a single
full-viewport `::before` pseudo-element to `.hf-header__row--nav` so a
continuous 1px line stretches edge-to-edge. The per-tab top borders sit
at the same Y-coordinate and overlap the pseudo-element where tabs
exist; the pseudo-element bridges the gaps elsewhere. The active-tab
brand-blue frame and z-index stacking are unchanged.

Author: GreenFlux

docs/src/styles/components/header.css

@@ -51,9 +51,11 @@ header.header {
 }
 
 /*
- * No border-bottom here on purpose — the framed-tab strip below already
- * supplies the visual divider via its 1px gray-5 border-top, so an additional
- * row-1 hairline would double-stack into a 2px line.
+ * The single horizontal divider that runs across the toolbar is rendered as
+ * a `::before` pseudo-element on `.hf-header__row--nav` below. The framed
+ * `.hf-nav__link` top borders overlap that line where tabs exist; the
+ * pseudo-element fills the gaps to the viewport edges. Mirrors HT's
+ * `.header-row-2::before`.
  */
 
 /* — Brand (logo + version pill) — */
@@ -138,6 +140,25 @@ header.header {
 /* — Primary nav (row 2) — */
 .hf-header__row--nav {
   background: var(--sl-color-bg-nav);
+  position: relative;
+}
+
+/*
+ * Continuous full-viewport divider line. Mirrors HT's `.header-row-2::before`.
+ * Each `.hf-nav__link` keeps its own 1px top border so the framed tabs read
+ * as a segmented strip; this pseudo-element sits at the same Y-coordinate
+ * and bridges the empty space to the left of the first tab and to the right
+ * of the Support dropdown so the divider reads as one unbroken rule.
+ */
+.hf-header__row--nav::before {
+  content: '';
+  position: absolute;
+  top: 0;
+  left: 50%;
+  transform: translateX(-50%);
+  width: 100vw;
+  height: 1px;
+  background: var(--sl-color-gray-5);
 }
 
 .hf-nav {