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/tailwind.config.ts

@@ -36,6 +36,7 @@ const config: Config = {
         graphite: 'var(--color-graphite)',
         // Fixed dark slab — never inverts in dark mode (utility strip, footer, dark bands)
         slab: '#111827',
+        'slab-hover': '#1f2937',
         'slab-text': '#f9fafb',
         'slab-muted': '#6b7280',
         // Semantic / decorative — fixed (not themeable)