fix(frontend): post-code-review NITs — btn-ink uses @apply bg-slab, unify New engagement label

- btn-ink: replace inline background-color #111827 with @apply bg-slab (and add
  slab-hover token #1f2937 for the hover state) so the token system is consistent
- EngagementsListPage: header button "+ New" → "+ New engagement" to match
  empty-state CTA label

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

frontend/src/pages/EngagementsListPage.tsx

@@ -42,7 +42,7 @@ export function EngagementsListPage(): JSX.Element {
         </div>
         {canEditEngagements ? (
           <Link to="/engagements/new" className="btn-primary">
-            <Plus size={14} aria-hidden /> New
+            <Plus size={14} aria-hidden /> New engagement
           </Link>
         ) : null}
       </header>