Move Studio Code Desktop from feature flag to beta flag

[sejas]

Proposed Changes

• Remove enableStudioCodeUi from the env-based FEATURE_FLAGS registry in apps/studio/src/lib/feature-flags.ts.
• Add enableStudioCodeUi to BetaFeatures in apps/studio/src/ipc-types.d.ts and register it in apps/studio/src/lib/beta-features.ts (label: "Studio Code Desktop", default off).
• Switch the consumer in apps/studio/src/components/content-tab-assistant.tsx from useFeatureFlags() to a Redux selector reading state.betaFeatures.features.enableStudioCodeUi.
• Seed the beta features slice initial state with enableStudioCodeUi: false and update the matching test fixtures in content-tab-settings.test.tsx.
• Enable the App menu's "Beta Features" submenu when at least one beta feature is registered (previously always disabled).

The ENABLE_STUDIO_CODE_UI env var is no longer wired. Users now toggle Studio Code Desktop from the macOS app menu under WordPress Studio → Beta Features → Studio Code Desktop. The choice persists in ~/.studio/app.json under betaFeatures.

Testing Instructions

• Start the app with npm start.
• Open WordPress Studio → Beta Features in the macOS menu bar — the "Studio Code Desktop" item should be present and unchecked by default.
• Open the Assistant tab on a site — the legacy WPCOM assistant should render.
• Toggle Studio Code Desktop on; reopen the Assistant tab — the new Studio Code chat UI should render instead.
• Toggle it back off; the legacy assistant should return.
• Quit and relaunch the app — the toggle state should persist (verify in ~/.studio/app.json under betaFeatures.enableStudioCodeUi).
• Confirm ENABLE_STUDIO_CODE_UI=true npm start no longer flips the UI (env var is intentionally removed).

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors? (Tests pass, no errors)