[codex] Move Desks UI switch to beta features

[youknowriad]

Related issues

• N/A

How AI was used in this PR

Codex made the targeted flag refactor, reviewed the diff, and ran the verification commands listed below.

Proposed Changes

• Move enableDesksUiSwitch out of env-backed dev feature flags.
• Add enableDesksUiSwitch to persisted beta features and expose it in the Beta Features menu.
• Read the Desks UI switch setting from beta-feature state in Preferences.
• Update beta-feature test state fixtures for the new key.

Testing Instructions

• npx eslint --fix apps/studio/src/components/tests/content-tab-settings.test.tsx apps/studio/src/components/tests/remote-session-indicator.test.tsx apps/studio/src/ipc-types.d.ts apps/studio/src/lib/beta-features.ts apps/studio/src/lib/feature-flags.ts apps/studio/src/modules/site-settings/tests/edit-site-details.test.tsx apps/studio/src/modules/user-settings/components/preferences-tab.tsx apps/studio/src/stores/beta-features-slice.ts
• npm run typecheck
• npm test -- apps/studio/src/components/tests/content-tab-settings.test.tsx apps/studio/src/modules/site-settings/tests/edit-site-details.test.tsx apps/studio/src/components/tests/remote-session-indicator.test.tsx

Pre-merge Checklist

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