[Web] Minify CSS

[markmur]

Web CSS lived in a css tagged template literal inside checkout.styles.ts, so it shipped verbatim and skipped Vite's minifier. Moving the styles into a real checkout.css file imported with ?inline lets Vite minify it, shaving ~1kb off the bundle. The now-unused css helper is dropped in favour of importing the stylesheet directly.

Verify the built bundle in platforms/web/dist still inlines the styles into the shadow template and that the overlay renders correctly in the web sample.

[github-actions]

Package Size

Platform	Artifact	Base	Head	Delta
Web	npm tarball	43.3 KiB	42.2 KiB	-1.1 KiB

Web file breakdown

File	Base	Head	Delta
dist/index.js.map	46.3 KiB	45.7 KiB	-571 B
dist/custom-elements.json	36.7 KiB	33.5 KiB	-3.3 KiB
src/checkout.ts	30.4 KiB	30.4 KiB	+20 B
README.md	16.4 KiB	16.4 KiB	0 B
dist/index.js	14.5 KiB	13.7 KiB	-807 B
src/ucp-embed-types.ts	9.6 KiB	9.6 KiB	0 B
dist/ucp-embed-types.d.ts	9.5 KiB	9.5 KiB	0 B
src/checkout.types.ts	7.4 KiB	7.4 KiB	0 B
dist/checkout.d.ts	7.0 KiB	7.0 KiB	0 B
dist/checkout.types.d.ts	6.1 KiB	6.1 KiB	0 B
src/checkout.styles.ts	2.8 KiB	—	-2.8 KiB
package.json	2.8 KiB	2.8 KiB	0 B
src/checkout.css	—	2.5 KiB	+2.5 KiB
src/utils.ts	1.7 KiB	1.6 KiB	-145 B
dist/utils.d.ts	1.4 KiB	1.2 KiB	-145 B
src/index.ts	1.2 KiB	1.2 KiB	0 B
LICENSE	1.1 KiB	1.1 KiB	0 B
dist/index.d.ts	749 B	749 B	0 B
src/checkout-web-component.ts	271 B	271 B	0 B
dist/checkout-web-component.d.ts	154 B	154 B	0 B
…and 3 smaller files

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.

[github-actions]

Web — Coverage Report

Lines	Statements	Branches	Functions
	98.49% (261/265)	88.48% (146/165)	100% (58/58)