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/components/Layout.tsx

@@ -59,8 +59,8 @@ export function Layout(): JSX.Element {
         </div>
       </div>
 
-      {/* nav-bar-top — canvas with hairline */}
-      <header className="bg-canvas border-b border-hairline">
+      {/* nav-bar-top — paper gives dark-mode lift vs canvas body */}
+      <header className="bg-paper border-b border-hairline">
         <div className="mx-auto w-full max-w-page px-xl h-16 flex items-center justify-between">
           <Link to="/engagements" className="flex items-center gap-sm" aria-label="Mimic home">
             <span className="inline-block h-6 w-6 rotate-12 bg-primary" aria-hidden />