Add Marketplace page and nav links

Introduce a new Marketplace page (packages/frontend/src/app/marketplace/page.tsx) with hero, stats, plugin categories, themes, user/dev workflows, revenue callout, FAQ, CTA and footer. Also update site navigation and footer across features, home, and pricing pages to include links to the Marketplace for discoverability.

Author: dewitt4

packages/frontend/src/app/features/page.tsx

@@ -128,7 +128,13 @@ export default function AboutPage() {
             href="/features"
             className="text-brand-600 font-medium border-b-2 border-brand-600 pb-0.5"
           >
-            About
+            Features
+          </Link>
+          <Link
+            href="/marketplace"
+            className="text-slate-600 hover:text-slate-900 font-medium"
+          >
+            Marketplace
           </Link>
           <Link
             href="/pricing"
@@ -445,6 +451,9 @@ export default function AboutPage() {
             <Link href="/features" className="hover:text-slate-900">
               Features
             </Link>
+            <Link href="/marketplace" className="hover:text-slate-900">
+              Marketplace
+            </Link>
             <Link href="/pricing" className="hover:text-slate-900">
               Pricing
             </Link>