feat(frontend): add BackLink helper + dedup 3 hand-rolled instances

BackLink: ArrowLeft (14px) + caption-md text-graphite hover:text-primary,
instant. Replaces inline Link patterns in EngagementDetailPage (already
committed), SimulationFormPage, and TemplateFormPage. Also migrates
SimulationFormPage Done/SOC banners to AlertBanner.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/BackLink.tsx

@@ -0,0 +1,17 @@
+import { Link } from 'react-router-dom';
+import { ArrowLeft } from 'lucide-react';
+import type { ReactNode } from 'react';
+
+interface BackLinkProps {
+  to: string;
+  children: ReactNode;
+}
+
+export function BackLink({ to, children }: BackLinkProps): JSX.Element {
+  return (
+    <Link to={to} className="inline-flex items-center gap-xxs caption-md text-graphite hover:text-primary">
+      <ArrowLeft size={14} aria-hidden />
+      {children}
+    </Link>
+  );
+}