Route storefront marketplace under /storefront

Replace the legacy /explore marketplace route with /storefront across the
storefront app and production routing.

- move the marketplace index page to the storefront root route
- move company and vehicle detail pages from /explore/* to /* internally
- update marketplace links, renter dashboard links, saved company links, and
  header navigation to target /storefront
- remove remaining explore route references and wording from storefront code
- configure production Traefik to route /storefront and strip the prefix before
  forwarding to the storefront service
- set the storefront production asset prefix to /storefront so Next chunks load
  from /storefront/_next instead of the shared root /_next path
- redirect locale entry paths such as /storefront/fr back to /storefront while
  persisting the selected language cookie
- update middleware tests for the locale redirect and adjusted redirect mock

Verification:
- npm run test --workspace @rentaldrivego/storefront
- npm run build --workspace @rentaldrivego/storefront

apps/storefront/next.config.js

@@ -8,10 +8,16 @@ const dashboardAssetSource = normalizeAssetPrefix(
   process.env.DASHBOARD_ASSET_PREFIX ?? (process.env.NODE_ENV !== 'production' ? 'http://localhost:3001' : undefined),
   '/dashboard',
 )
+const STOREFRONT_ASSET_PREFIX = '/storefront'
+const storefrontAssetPrefix = normalizeAssetPrefix(
+  process.env.STOREFRONT_ASSET_PREFIX ?? (process.env.NODE_ENV === 'production' ? STOREFRONT_ASSET_PREFIX : undefined),
+  STOREFRONT_ASSET_PREFIX,
+)
 const securityHeaders = buildSecurityHeaders({
-  assetSources: [dashboardAssetSource, process.env.ADMIN_ASSET_PREFIX],
+  assetSources: [storefrontAssetPrefix, dashboardAssetSource, process.env.ADMIN_ASSET_PREFIX],
 })
 const nextConfig = {
+  ...(storefrontAssetPrefix ? { assetPrefix: storefrontAssetPrefix } : {}),
   images: {
     remotePatterns: [
       {