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>

templates/404.tpl

@@ -0,0 +1,12 @@
+<div class="flex-fill d-flex flex-column justify-content-center">
+	<h2 class="fw-semibold tracking-tight text-center">[[westgate:404.title]]</h2>
+
+	<div class="mx-auto">
+		<div class="d-flex flex-column gap-3 justify-content-center text-center">
+			<div class="mx-auto p-4">
+				<i class="text-secondary fa fa-fw fa-4x {{{ if icon }}}{icon}{{{ else }}}fa-mask{{{ end }}}"></i>
+			</div>
+			<p class="mb-0">{{{ if error }}}{tx(error)}{{{ else }}}[[westgate:404.message, {config.relative_path}]]{{{ end }}}</p>
+		</div>
+	</div>
+</div>