[Web] Use a button for checkout overlay focus

[markmur]

What changes are you making?

• Replace the overlay anchor with a native button because its action is to focus the already-open checkout window, not navigate.
• Remove the overlay URL and href update logic, preventing browser-managed tabs from loading an embedded checkout without a transport.
• Preserve the link-like visual treatment while providing native button keyboard semantics.
• Define CHECKOUT_KIT_PACKAGE_VERSION in the sample Vite build so direct source imports run correctly.

Fixes https://github.com/shop/issues/issues/74036
Related to https://github.com/shop/issues/issues/77405

[github-actions]

Web — Coverage Report

Lines	Statements	Branches	Functions
	94.4% (287/304)	81.11% (146/180)	98.7% (76/77)

[github-actions]

Package Size

Platform	Artifact	Base	Head	Delta
Web	npm tarball	70.6 KiB	70.2 KiB	-350 B

Web file breakdown

File	Base	Head	Delta
dist/index.js.map	192.2 KiB	191.6 KiB	-594 B
dist/index.d.ts	44.9 KiB	44.9 KiB	0 B
dist/custom-elements.json	44.0 KiB	43.8 KiB	-279 B
dist/index.js	31.5 KiB	31.5 KiB	-31 B
README.md	19.0 KiB	19.0 KiB	0 B
package.json	2.8 KiB	2.8 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.