Fix header gradient too light in dark mode

Add dedicated header-from/header-to color tokens that are independent
of the primary palette dark mode overrides. Dark mode header uses a
deeper indigo gradient instead of the brightened primary colors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

resources/scripts/admin/layouts/partials/TheSiteHeader.vue

@@ -13,8 +13,8 @@
       py-3
       md:h-16 md:px-8
       bg-linear-to-r
-      from-primary-500
-      to-primary-400
+      from-header-from
+      to-header-to
     "
   >
     <router-link