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/src/components/WorkspaceFrame.boundary.test.ts

@@ -29,7 +29,7 @@ describe('WorkspaceFrame helpers', () => {
   })
 
   it('keeps unauthenticated visitors on the embedded sign-in path', () => {
-    stubBrowser({ cookie: 'marketplace-language=fr' })
+    stubBrowser({ cookie: 'storefront-language=fr' })
 
     expect(getDefaultFramePath('sign-in')).toBe('/dashboard/sign-in')
   })