Upgrade @11ty/is-land from v4 to v5

Summary

Dependabot opened PR #2120 to bump @11ty/is-land from 4.0.1 to 5.0.1. This is a major version bump with breaking changes, and the CI build is currently failing on that PR. Parking this as a tracked issue rather than merging blindly.

Breaking Changes in v5

• autoinit attribute removed — <is-land autoinit> and is-land-autoinit.js are gone. Must migrate to the new Island.addInitType() API for framework initialisation
• Island.define(tagName) removed — replaced by using customElements.define directly
• import attribute behaviour change — when used with type, the import() is no longer performed automatically
• Manual definition — now uses ?nodefine query param instead of the previous approach

What's Good

• Broader browser support (Chrome 71+, down from 88+)
• Performance improvements for on:idle, on:media, on:load
• Passive event listeners for on:interaction (fixes Lighthouse warning)
• Minified build available (dist/is-land.min.js)

Suggested Approach

1. Audit current is-land usage in templates (src/_includes/, src/_layouts/)
2. Check if autoinit or Island.define() are used anywhere
3. Update usage to match v5 API
4. Test lazy-loading behaviour (theme switcher, search, any other islands)
5. Verify build passes

References

• is-land v5.0.0 release
• Dependabot PR #2120