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/_surfaces.scss

@@ -5,6 +5,10 @@ body {
 }
 
 body {
+  --bs-link-color: var(--wg-gold);
+  --bs-link-color-rgb: 194, 163, 90;
+  --bs-link-hover-color: #e0c878;
+  --bs-link-hover-color-rgb: 224, 200, 120;
   font-family: var(--wg-font-text);
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
@@ -14,6 +18,12 @@ body {
   background: var(--wg-bg);
 }
 
+// h-100 on #content collapses inside the flex-grown #panel, so the 404's
+// flex-fill never gets height to center in; let #content grow instead
+body.template-404 #content {
+  flex-grow: 1;
+}
+
 .layout-container,
 #content {
   color: var(--wg-text-soft);