ui: add custom centered 404 page

internal/templates/not_found.templ

@@ -0,0 +1,12 @@
+package templates
+
+templ NotFoundPage() {
+	@Layout("mal - not found", false) {
+		<section class="not-found-page anime-surface">
+			<p class="not-found-code">404</p>
+			<h1>Page not found</h1>
+			<p class="empty-inline-note">The page you requested does not exist, or it was moved.</p>
+			<p><a href="/" class="not-found-link">Back to catalog</a></p>
+		</section>
+	}
+}