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/tailwind.config.ts

@@ -36,7 +36,6 @@ 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)