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/api/src/modules/admin/admin.routes.ts

@@ -559,16 +559,16 @@ router.post('/subscriptions/:subscriptionId/cancel', requireAdminAuth, requireAd
 
 // ─── Site config ───────────────────────────────────────────────
 
-router.get('/site-config/marketplace-homepage', requireAdminAuth, async (req, res, next) => {
+router.get('/site-config/storefront-homepage', requireAdminAuth, async (req, res, next) => {
   try {
-    ok(res, await service.getMarketplaceHomepage())
+    ok(res, await service.getStorefrontHomepage())
   } catch (err) { next(err) }
 })
 
-router.patch('/site-config/marketplace-homepage', requireAdminAuth, requireAdminRole('SUPPORT'), async (req, res, next) => {
+router.patch('/site-config/storefront-homepage', requireAdminAuth, requireAdminRole('SUPPORT'), async (req, res, next) => {
   try {
     const { homepage } = parseBody(homepageUpdateSchema, req)
-    ok(res, await service.updateMarketplaceHomepage(homepage, req.admin.id, req.ip))
+    ok(res, await service.updateStorefrontHomepage(homepage, req.admin.id, req.ip))
   } catch (err) { next(err) }
 })