Flat purple panels, themed 404, global gold links (#23)

- Replaces the diagonal plum/velvet panel gradients with flat subtle purples (anchored to the group-card tone `rgb(24,20,29)`); the topbar uses the same flat tone so it meshes with the panels.
- Adds a Westgate-themed 404 page ("You've wandered off the map." / Night Masks warning) with a mask icon, vertically centered in the content area. Strings live in `languages/en-GB/westgate.json`.
- Sets Bootstrap link CSS variables globally so utility-classed links (`alert-link`, `link-*`) get the wiki gold like plain anchors already did. Topic-title parchment override is untouched.

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Reviewed-on: #23
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.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>