Split the WordPress version setting into an update mode and a version picker

[gcsecsey]

Related issues

• Related to STU-2348
• Stacked on Accept --wp auto-update as the CLI name for the auto-update mode #4750
• Lifts the update-mode control from Design exploration: Clarify Agentic UI site settings #4747

How AI was used in this PR

For implementation and tests: lifting the control from #4747, fixing the two edge cases below, and updating the affected tests.

Proposed Changes

The WordPress version setting mixes two decisions: whether the site updates itself, and which version it is pinned to. #4702 fixes the wording of the single dropdown, but the two decisions still share one control. This splits them into Automatic updates and Select a version, so the picker only means "pin this version". The interaction and copy come from Shaun's exploration in #4747; this PR takes only that control, not the wider settings redesign in that PR.

• A pinned site whose installed version Studio cannot read now shows Unknown version in the picker. Before, the picker had no option for that state and silently displayed the newest offer instead.
• Without a version list (no network, no cached offers) the field stays a plain dropdown, because Select a version would open an empty picker.
• The installed version only appears under Automatic updates while that mode is selected, which is the rule Set the right expectation for the "latest" WordPress version setting #4702 uses for its bracketed label.

Testing Instructions

• Run npm run cli:build:ui && node apps/cli/dist/cli/main.mjs ui --no-open, then open a site and its Settings tab.
• An auto-updating site shows Automatic updates selected, and its description names the version the site runs now.
• Select Select a version, pick a different version, and confirm Save settings becomes enabled.
• Repeat in light and dark appearance (App settings, Appearance).

State	Light	Dark
Automatic updates	Cell	Cell
Select a version	Cell	Cell

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?