fix(frontend): sprint 4 design-review — slab token + UsersAdmin alignment + dark hairlines + badge contrast

- bump dark hairline from #374151 → #4b5563 for visible table borders
- topbar header bg-canvas → bg-paper for dark-mode lift vs canvas body
- UsersAdminPage create-form: Option A structural 3-row grid (labels / inputs / hints)
  to fix AC-17.3 alignment; removes FormField wrapper that caused row-height misalignment
- EngagementsListPage: replace text "+ New" with lucide Plus icon per design spec

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

frontend/src/pages/EngagementsListPage.tsx

@@ -1,4 +1,5 @@
 import { Link } from 'react-router-dom';
+import { Plus } from 'lucide-react';
 import { extractApiError } from '@/api/client';
 import type { Engagement } from '@/api/types';
 import { useDeleteEngagement, useEngagementsList } from '@/hooks/useEngagements';
@@ -41,7 +42,7 @@ export function EngagementsListPage(): JSX.Element {
         </div>
         {canEditEngagements ? (
           <Link to="/engagements/new" className="btn-primary">
-            + New
+            <Plus size={14} aria-hidden /> New
           </Link>
         ) : null}
       </header>
@@ -59,7 +60,7 @@ export function EngagementsListPage(): JSX.Element {
           action={
             canEditEngagements ? (
               <Link to="/engagements/new" className="btn-primary">
-                + New engagement
+                <Plus size={14} aria-hidden /> New engagement
               </Link>
             ) : undefined
           }