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/src/app/onboarding/page.tsx

@@ -271,7 +271,7 @@ export default function OnboardingPage() {
                   checked={payments.isListedOnMarketplace}
                   onChange={(e) => setPayments({ ...payments, isListedOnMarketplace: e.target.checked })}
                 />
-                <span className="text-sm text-slate-700">List my company on the RentalDriveGo marketplace</span>
+                <span className="text-sm text-slate-700">List my company on the RentalDriveGo storefront</span>
               </label>
               <div className="flex gap-3">
                 <button onClick={() => setStep(2)} className="btn-secondary flex-1 justify-center">Back</button>