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>

languages/en-GB/westgate.json

@@ -1,3 +1,5 @@
 {
-	"wiki-badge": "Wiki"
+    "wiki-badge": "Wiki",
+    "404.title": "You've wandered off the map.",
+    "404.message": "This alley leads nowhere, and in Westgate, nowhere is where the Night Masks find you.<br>Turn back to the <a href=\"%1/\">lantern-lit streets</a> before they notice you lingering."
 }