[codex] Fix preview environment selection highlight

[JamesSprow]

Summary

Fix the project navbar environment switcher so PR preview environments stay visually selected when they are the active environment.

Root cause

The layout-level currentEnv derived state only treated declared project environments as valid. When the stored environment was a preview environment, the UI fell back to the first regular environment for rendering, so the preview was active in state and the URL but not shown as selected.

What changed

• accept preview environment names as valid current environment values in the top-level layout
• preserve existing fallback behavior only when the stored environment matches neither a regular environment nor a preview environment

Validation

• npm run check in ui