Add Magento commerce app port to @decocms/apps-start

Summary

@decocms/apps-start ships VTEX and Shopify ports today (per the package README and folder layout). There is no Magento port, even though deco-cx/apps has a mature Magento integration that powers multiple production sites.

This blocks migration of Magento-based Fresh storefronts to TanStack Start: deco-migrate deletes the legacy magento/ folder's imports but the new src/setup/commerce-loaders.ts has nothing to map them to, so cart / user / wishlist / PDP / PLP loaders break post-migration.

Source to port

deco-cx/apps, folder magento/ — 60 TypeScript files, structurally identical to apps/vtex/ (mod.ts, manifest.gen.ts, loaders/, actions/, hooks/, middleware.ts, utils/clientGraphql, utils/client, utils/transform.ts, utils/cache.ts).

Key entry points to port (≈54 generic files):

• mod.ts — App definition with configureMagento + middleware
• loaders/cart.ts, loaders/user.ts, loaders/wishlist.ts, loaders/proxy.ts
• loaders/product/detailsPage.ts, loaders/product/listingPage.ts, loaders/product/list.ts
• loaders/routes/*
• actions/{cart,newsletter,product,wishlist}/*
• hooks/{useCart,useUser,useWishlist}.ts (refactor to react-query like vtex/hooks/)
• utils/clientGraphql/*, utils/client/*, utils/{transform,cache,graphql,cart}.ts

Required adaptations (Deno → Node)

• $fresh/server.ts imports → TanStack/React equivalents
• deco/* imports → @decocms/start/* and @decocms/apps-start/commerce
• Drop .ts extension from relative imports
• Replace Deno std/cache LRU with npm:lru-cache (mirror pattern in apps-start/vtex/utils/fetchCache.ts)
• Wrap loaders/actions in createServerFn
• Cookies via @decocms/apps-start/commerce helpers

CRITICAL: preserve ExtensionOf pattern

The existing Magento app accepts extensions?: ExtensionOf<T>[] on key loaders (loaders/extensions/cart, loaders/extensions/product/{detailsPage,listingPage,list}). This is how sites plug in third-party features (Livelo points, Amasty reviews, etc.) without forking the app.

Please port this pattern intact. The first consumer (granadobr-tanstack) will keep Granado-specific extensions (Livelo + Amasty) in src/apps/magento-granado/ locally — that only works if the generic Magento loaders accept and apply extensions[].

Context

Sites blocked on this:

• granadobr-tanstack (Granado, Brazilian cosmetics) — repo: deco-sites/granadobr-tanstack
• Future Magento migrations across the deco fleet

Companion framework issue: decocms/deco-start — Platform magento missing from migrate's enum.

Happy to send a PR — let me know the preferred patch granularity (one big PR vs split per area).

Compare the source deco-cx/apps/magento/ folder with the existing apps/vtex/ port, starting at mod.ts, the commerce loaders, actions, hooks, and utilities named in the issue. Check the TanStack/Node adaptations and apps-start/vtex/utils/fetchCache.ts first. Done means a Magento app port works in @decocms/apps-start, including the ExtensionOf pattern for cart and product loaders.