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

storefront/next.config.js

@@ -2,7 +2,7 @@
 
 const { buildSecurityHeaders, normalizeAssetPrefix } = require('../../config/nextSecurityHeaders')
 
-// The marketplace proxies /dashboard and /admin to their own Next dev servers.
+// The storefront proxies /dashboard and /admin to their own Next dev servers.
 // Allow those asset-prefix origins so proxied pages can load chunks and HMR.
 const dashboardAssetSource = normalizeAssetPrefix(
   process.env.DASHBOARD_ASSET_PREFIX ?? (process.env.NODE_ENV !== 'production' ? 'http://localhost:3001' : undefined),