fix(frontend): sprint 4 post-code-review — btn-ink uses slab token + unify New CTA label

- btn-ink hover: bg-slab-hover (unnecessary new token) → bg-paper (existing token,
  same #1f2937 value in dark, avoids token sprawl)
- tailwind.config.ts: remove slab-hover token added in fc530af
- EngagementsListPage: both CTAs unified to "+ New" (≤8 chars convention, AC-17.2)

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 engagement
+            <Plus size={14} aria-hidden /> New
           </Link>
         ) : null}
       </header>
@@ -60,7 +60,7 @@ export function EngagementsListPage(): JSX.Element {
           action={
             canEditEngagements ? (
               <Link to="/engagements/new" className="btn-primary">
-                <Plus size={14} aria-hidden /> New engagement
+                <Plus size={14} aria-hidden /> New
               </Link>
             ) : undefined
           }