[Web] Playwright synthetic E2E tests

[markmur]

What changes are you making?

Adds a hermetic Playwright end-to-end suite for the web component.

• Adds a standalone e2e/web package with Playwright, a local static fixture server, and synthetic checkout fixtures.
• Runs the built platforms/web/dist/index.js in Chromium so the tests exercise the packaged custom element rather than source-only unit-test behavior.
• Covers custom element registration, overlay/link URL behavior, popup open/close, protocol notifications, completion, unrecoverable errors, and ec.window.open_request delegation.
• Adds dev web e2e and includes the web e2e suite in dev web check.
• Wires Web CI to install the e2e package, install Playwright Chromium, typecheck the e2e tests, and run the suite after the web package build.
• Updates the e2e README with the web suite layout, scope, and local run commands.

How to test

dev web e2e
pnpm --dir e2e/web typecheck

Also verified:

git diff --check

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Before you merge

Important

• I've added tests to support my implementation
• I have read and agree with the Contribution Guidelines
• I have read and agree with the Code of Conduct
• I've updated the relevant platform README (platforms/swift/README.md and/or platforms/android/README.md)

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Releasing a new Swift version?

• I have bumped the version in ShopifyCheckoutKit.podspec
• I have bumped the version in platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift
• I have updated the SwiftPM/CocoaPods version snippets in platforms/swift/README.md (major version only)

Releasing a new Embedded Checkout Protocol version?

• I have bumped embeddedCheckoutProtocolAndroid in platforms/android/gradle/libs.versions.toml
• I have updated protocol/languages/kotlin/embedded-checkout-protocol/api/embedded-checkout-protocol.api if the public API changed

Releasing a new Android version?

• I have bumped checkoutKitAndroid in platforms/android/gradle/libs.versions.toml
• I have updated the Gradle/Maven version snippets in platforms/android/README.md

Tip

See the Contributing documentation for the full release process per platform.