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

PHASE16_DESIGN_MIGRATION_MANIFEST.json

@@ -49,13 +49,13 @@
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     },
     {
-      "path": "storefront/src/components/MarketplaceFooter.tsx",
+      "path": "storefront/src/components/StorefrontFooter.tsx",
       "status": "modified",
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       "sha256": "169d04dcf0976d2637ef5371ca295e9c362bd089d8d6ec313b4e8b4131d5bdd3"
     },
     {
-      "path": "storefront/src/components/MarketplaceHeader.tsx",
+      "path": "storefront/src/components/StorefrontHeader.tsx",
       "status": "modified",
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       "sha256": "25afc57ff9893f255eddf19580e84b510e6032764daaa5727e2e8d669dca4213"