feat(web): report unsupported browser capabilities

[kiftio]

What changes are you making?

• Detect missing Shadow DOM, native dialog, and abortable event-listener support before opening checkout.
• Emit the existing non-bubbling error event with detail.error.code === "unsupported_browser", without opening a checkout window.
• Document the fallback pattern and cover the event behavior.

How to test

• dev web test
• dev web lint

Stacked on #860.

[github-actions]

Web — Coverage Report

Lines	Statements	Branches	Functions
	94.31% (398/422)	83.6% (209/250)	96.29% (104/108)

[github-actions]

Size budgets

Measured head: 3cf8a4293dec10a230e410accb27160499943adf; base: 0d8f23739a4d8d4aeb4c171005c3432d8f9d5995.

Platform / budget	Measurement	Base	Head	Delta	Soft	Hard	Result
web / javascript	Bundle (uncompressed)	33.27 KiB	34.06 KiB	+0.78 KiB	35 KiB	50 KiB	Within budget

Repository writers, including the PR author, can accept current soft-budget breaches with a reason:

/accept-size web Explain why this increase is necessary.

Use one command per platform; several lines can share a comment. Post after this report is ready for the current head. Commands in edited comments are not accepted.

Acceptance applies to each currently exceeded metric up to its recorded size. Further growth or a newly exceeded metric needs fresh acceptance. Hard-budget increases require a reviewed change to .ci/bundle-size-budgets.json.

Bundle and package size

Web bundle sizes cover shipped runtime JavaScript. Package sizes cover the full published archive, including any source maps, declarations, and documentation it contains.

Platform	Measurement	Compression	Base	Head	Delta
Web	JavaScript bundle	Uncompressed	33.3 KiB	34.1 KiB	+800 B
Web	JavaScript bundle	gzip	10.2 KiB	10.4 KiB	+231 B
Web	npm package (.tgz)	gzip	89.5 KiB	91.0 KiB	+1.4 KiB

Web package files (uncompressed)

These are uncompressed file sizes; they do not sum to the compressed package size above.

File	Base	Head	Delta
dist/index.js.map	249.0 KiB	252.1 KiB	+3.1 KiB
dist/custom-elements.json	47.5 KiB	48.8 KiB	+1.3 KiB
dist/index.d.ts	47.9 KiB	48.0 KiB	+46 B
dist/index.js	33.3 KiB	34.1 KiB	+800 B
README.md	20.2 KiB	20.8 KiB	+544 B
package.json	3.6 KiB	3.6 KiB	0 B
LICENSE	1.1 KiB	1.1 KiB	0 B

Measured from the PR base SHA and PR head SHA. Web bundle rows sum shipped .js, .mjs, and .cjs files under dist/, excluding source maps and declarations. The gzip bundle size sums files compressed individually with gzip -n -9. npm package sizes are gzip-compressed .tgz archives; Android AAR sizes are ZIP archives. Package sizes are not final app binary sizes.