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/pages/TemplateFormPage.tsx

@@ -9,6 +9,7 @@ import { FormField, TextArea, TextInput } from '@/components/FormField';
 import { LoadingState } from '@/components/LoadingState';
 import { ErrorState } from '@/components/ErrorState';
 import { ConfirmDialog } from '@/components/ConfirmDialog';
+import { BackLink } from '@/components/BackLink';
 import { MitreTechniqueTag, MitreTacticTag } from '@/components/MitreTechniqueTag';
 import { MitreTechniquePicker } from '@/components/MitreTechniquePicker';
 import { MitreMatrixModal } from '@/components/MitreMatrixModal';
@@ -127,9 +128,7 @@ export function TemplateFormPage(): JSX.Element {
     <div className="flex flex-col gap-xl">
       <header className="flex items-start justify-between gap-md">
         <div className="flex flex-col gap-sm">
-          <Link to="/admin/templates" className="btn-text-link text-[14px]">
-            ← Back to templates
-          </Link>
+          <BackLink to="/admin/templates">Back to templates</BackLink>
           <h1 className="text-[32px] font-medium leading-none">
             {isNew ? 'New template' : (existing.data?.name ?? 'Edit template')}
           </h1>