fix(theme): paint the navbar divider on its own layer

Safari composites the sticky local nav above the bar's negative-z
rule in their overlapping row, regardless of z-index, hiding the
divider below 60rem (#5399). Promoting the rule to its own layer
restores the order. Flyout panels still cover it.

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

src/client/theme-default/components/VPNavBar.vue

@@ -274,6 +274,9 @@ const overflow = provideNavOverflow({
   /* above the background surface, below the bar's content — an open flyout
      panel overlaps the bar's bottom edge and must cover the rule */
   z-index: -1;
+  /* own layer — Safari otherwise sorts the rule behind the sticky local
+     nav's surface in their overlapping row (#5399) */
+  transform: translateZ(0);
   width: 100%;
   height: 1px;
   padding-left: var(--vp-nav-col-offset);