Flatten panel gradients to subtle purple, themed 404, gold links

- Replace the velvet/ledger diagonal plum gradients with one flat
  purple per surface (anchored to the group-card tone rgb(24,20,29));
  topbar now uses the same flat tone so it meshes with the panels
- Add a Westgate-themed 404 page (Night Masks warning, fa-mask icon),
  vertically centered in the content area; strings in westgate.json
- Set Bootstrap link CSS vars globally so utility-classed links
  (alert-link, link-*) get the wiki gold like plain anchors

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

scss/westgate/_controls.scss

@@ -397,7 +397,7 @@ table[component="notification/table"] a[data-type]:focus {
       transparent calc(100% - 1px),
       rgba(194, 163, 90, 0.055) calc(100% - 1px)
     ),
-    linear-gradient(100deg, rgba(32, 19, 30, 0.98), rgba(13, 12, 17, 0.98)) !important;
+    var(--wg-velvet-panel) !important;
   background-size:
     100% var(--wg-ledger-line),
     auto;