Add browser coverage for human-readable routes

[snowyukitty]

Type of Change

• Utilities: Normalize Vite module IDs while resolving island entry points.
• Tests: Add Vitest Browser/Playwright coverage for human-readable API and library-version output.
• Something else: Install Chromium and run the browser suite in CI.

What issue does this relate to?

Fixes #188

What should this PR do?

• Run browser tests against a real local Wrangler worker through a same-origin Vite proxy.
• Assert the /api?output=human documentation and /libraries/backbone.js/1.1.0?output=human version page with accessible roles and labels.
• Exercise the hydrated Files island by selecting the displayed Source Maps filter and checking the resulting file links.
• Keep browser specs separate from the Workers test pool and allocate local ports dynamically.
• Normalize Vite module IDs so island bundling works with Windows paths.

What are the acceptance criteria?

• npm test
• npm run test:browser
• npm run lint
• npm run types
• npx wrangler deploy --dry-run

This adds test coverage and cross-platform build normalization; it does not change the rendered UI, so before/after screenshots are not applicable.

[MattIPv4]

Thanks for working on this! Instead of unstable_dev, I wonder if we can use the new createTestHarness designed for use w/ Playwright etc.? https://developers.cloudflare.com/changelog/post/2026-07-21-integration-test-harness/

[MattIPv4]

Instead of a dedicated spec file for the human outputs, I think it'd be good to include this as part of the existing spec files for each route, if possible? Or, if not, at least have a dedicated browser spec file alongside each existing route/spec?