Extract shared Web checkout presentation

[tiagocandido]

What changes are you making?

Extract Web checkout presentation into shared presentation helpers so the standard and Universal elements can share popup, overlay, focus and close behavior. Preserve the standard element's protocol-neutral events and Kit-owned snapshot mapping.

Reset presentation state before opening, reject messages from a null or unrelated sender, and isolate queued dialog-close callbacks from a newly opened presentation. Keep appearance, popup fallback and telemetry navigation timing intact.

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

The full stack passes 440 Web tests plus lint/typechecks, build, package verification and sample build. Regression coverage includes close/reopen, blocked popups, sender/origin isolation, presentation reset, and standard lifecycle events.

[github-actions]

Web — Coverage Report

Lines	Statements	Branches	Functions
	95.18% (376/395)	85.77% (199/232)	97.34% (110/113)

[github-actions]

Package Size

Platform	Artifact	Base	Head	Delta
Web	npm tarball	90.9 KiB	91.6 KiB	+718 B

Web file breakdown

File	Base	Head	Delta
dist/index.js.map	249.3 KiB	250.0 KiB	+746 B
dist/custom-elements.json	48.0 KiB	55.1 KiB	+7.2 KiB
dist/index.d.ts	47.7 KiB	47.7 KiB	0 B
dist/index.js	41.8 KiB	42.2 KiB	+465 B
README.md	20.1 KiB	20.1 KiB	0 B
package.json	2.9 KiB	2.9 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.