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/_widgets.scss

@@ -54,7 +54,7 @@
 	padding: var(--wg-space-md);
 	background:
 		linear-gradient(to bottom, transparent calc(100% - 1px), rgba(194, 163, 90, 0.045) calc(100% - 1px)),
-		linear-gradient(100deg, rgba(42, 18, 34, 0.34), rgba(18, 16, 23, 0.96) 62%, rgba(13, 12, 17, 0.98)) !important;
+		var(--wg-velvet-panel) !important;
 	background-size: 100% var(--wg-ledger-line), auto;
 	border: 1px solid rgba(194, 163, 90, 0.14);
 	border-radius: 8px;