refactor: rename marketplace to storefront across the entire monorepo

Comprehensive rename of all marketplace references to storefront:
- API module: apps/api/src/modules/marketplace/ → storefront/
- Components: MarketplaceHeader → StorefrontHeader, MarketplaceShell →
  StorefrontShell, MarketplaceFooter → StorefrontFooter
- Types: marketplace-homepage.ts → storefront-homepage.ts
- Test files: employee-marketplace-* → employee-storefront-*
- All source code identifiers, imports, route paths, and strings
- Documentation (docs/), CI config (.gitlab-ci.yml), scripts
- Dashboard, admin, storefront workspace references
- Prisma field names preserved (isListedOnMarketplace, marketplaceRating,
  marketplaceFunnelEvent) as they map to database schema

Validation:
- API type-check: 0 errors
- Storefront type-check: 0 errors
- Dashboard type-check: 0 errors
- Full monorepo type-check: only pre-existing admin TS18046

apps/dashboard/next.config.js

@@ -30,9 +30,9 @@ const DASHBOARD_BASE_PATH = '/dashboard'
 // basePath is required so the client-side router knows it's mounted at /dashboard.
 // Without it, React cannot hydrate because the URL (/dashboard/sign-up) doesn't
 // match the route (/sign-up). The Turbopack double-basePath prefetch bug is
-// handled by the marketplace middleware (307 redirect /dashboard/dashboard → /dashboard).
+// handled by the storefront middleware (307 redirect /dashboard/dashboard → /dashboard).
 //
-// In local development the dashboard is often viewed through the marketplace
+// In local development the dashboard is often viewed through the storefront
 // proxy on port 3000 while the dashboard dev server runs on port 3001. Next
 // rewrites do not reliably proxy HMR WebSocket upgrades, so emit absolute
 // dashboard asset/HMR URLs directly to the dashboard dev server.