Add multi-shop Universal Checkout sample carts

[tiagocandido]

What changes are you making?

Add a separate Universal Checkout sample page with editable storefront domains, product loading, per-shop quantities and independent cart previews. Keep the existing single-checkout page available through shared navigation.

The state/controller/view split handles stale catalog responses, failed-shop retry, unavailable variants, cart invalidation and page restoration. Layouts keep all shop catalogs and controls reachable at wide and narrow widths. This layer builds the cart UI; session creation and checkout presentation are added above it in the stack.

How to test

From the repository root, with dependencies installed:

pnpm --dir platforms/web lint
pnpm --dir platforms/web test
pnpm --dir platforms/web build
pnpm --dir platforms/web verify
pnpm --dir platforms/web sample:build

Start the sample with pnpm --dir platforms/web sample and open /universal.html. Add two eligible storefront domains, select different products and quantities, remove/re-add a shop, retry a failed catalog and resize the page. The full stack passes 440 Web tests and sample lint/typecheck/build; focused tests cover domain parsing, asynchronous catalogs, quantities, availability and the rendered cart controls.

tophat results

On the public-base revision, two shops loaded from ignored environment configuration. Both catalogs loaded and independent product selections populated their carts. Removing a shop invalidated the prepared URL; reloading restored the configured shops with empty carts.

[github-actions]

Web — Coverage Report

Lines	Statements	Branches	Functions
	87.96% (753/856)	82.72% (479/579)	92.18% (177/192)

[github-actions]

Package Size

Platform	Artifact	Base	Head	Delta
Web	npm tarball	126.3 KiB	126.3 KiB	0 B

Web file breakdown

File	Base	Head	Delta
dist/telemetry-BhYuVVw_.js.map	215.7 KiB	215.7 KiB	0 B
dist/custom-elements.json	90.8 KiB	90.8 KiB	0 B
dist/universal.js.map	63.9 KiB	63.9 KiB	0 B
dist/index.d.ts	47.7 KiB	47.7 KiB	0 B
dist/universal.d.ts	44.7 KiB	44.7 KiB	0 B
dist/index.js.map	34.8 KiB	34.8 KiB	0 B
dist/telemetry-BhYuVVw_.js	34.4 KiB	34.4 KiB	0 B
README.md	20.1 KiB	20.1 KiB	0 B
dist/universal.js	18.9 KiB	18.9 KiB	0 B
dist/index.js	8.3 KiB	8.3 KiB	0 B
package.json	3.1 KiB	3.1 KiB	0 B
LICENSE	1.1 KiB	1.1 KiB	0 B

Measured from the PR base SHA and PR head SHA. The file breakdown shows uncompressed sizes within each package artifact, so individual files do not sum to the compressed artifact total. This comment reports package artifact sizes only; it is not a final app binary-size report.