feat(theme): add --vp-local-nav-divider-color

The local nav's bottom rule reads its own token instead of a
hardcoded gutter color. It follows --vp-nav-divider-color by default,
like the surface color follows the navbar's.

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

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

@@ -71,7 +71,7 @@ const isScrolled = computed(() => y.value >= navHeight.value)
   /*rtl:ignore*/
   left: 0;
   z-index: var(--vp-z-index-local-nav);
-  border-bottom: 1px solid var(--vp-c-gutter);
+  border-bottom: 1px solid var(--vp-local-nav-divider-color);
   padding-top: var(--vp-layout-top-height, 0px);
   width: 100%;
 }

src/client/theme-default/styles/vars.css

@@ -536,6 +536,12 @@
    * frosted-glass treatment to the local nav as well.
    */
   --vp-local-nav-bg-color: var(--vp-nav-bg-color);
+
+  /**
+   * The local nav's bottom rule. Follows the navbar's divider color by
+   * default.
+   */
+  --vp-local-nav-divider-color: var(--vp-nav-divider-color);
 }
 
 /**