Allow pinning Default UI Version to the newest release without automatic upgrades

🔍 Problem Statement

As a site admin, I can only adopt the newest UI version by selecting "Latest," which results in automatic future upgrades without consent.

💡 User Story

• As a site admin, I want to pin my site to the newest UI version, so that I get current features without automatic future upgrades.

• As a site admin, I want "Latest" to remain available, so that I can still opt into automatic upgrades.

🎯 Definition of Done (DoD)

• ✔ Given the Default UI Version setting, when an admin opens the dropdown, then the newest release (currently 3.0) is listed as a pinnable option separate from "Latest (3.0)".

• ✔ Given a site pinned to 3.0, when a newer UI version is released, then the site continues serving 3.0 until an admin changes the setting.

• ✔ Given a site set to "Latest", when a newer UI version is released, then the site serves the new version, unchanged from current behavior.

• ✔ Given a new UI version is released, when it is published, then it appears in the dropdown as a pinnable option without manual configuration.

• ✔ Given a site settings API request, when the UI version is read or updated, then the API accepts and returns either a specific version or "latest".

• ✔ This change will impact UI, API, and SDK, specifically the Site Settings Default UI Version dropdown, [clarify site settings endpoint/field], and the UI bundle version loader.

• ✔ This solution will include the following limitations: no notification of new UI releases; no automatic migration of existing "Latest" sites to a pinned version; no per-page or per-environment version selection.