Move Studio Code Desktop from feature flag to beta flag

[sejas]

Proposed Changes

• Let's move Studio Code UI to beta flags so users can enable it in their end.

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 Electron → 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)

[youknowriad]

What do you think about moving it to Stable directly for the next release (I'm working on metrics and I'll have a PR for that soon)?

[youknowriad]

Approving but I'm wondering if we can go directly to stable.

[wpmobilebot]

📊 Performance Test Results

Comparing b7e2455 vs trunk

app-size

Metric	trunk	b7e2455	Diff	Change
App Size (Mac)	1329.74 MB	1329.74 MB	0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	b7e2455	Diff	Change
load	1682 ms	1621 ms	61 ms	🟢 -3.6%

site-startup

Metric	trunk	b7e2455	Diff	Change
siteCreation	9067 ms	9028 ms	39 ms	⚪ 0.0%
siteStartup	4405 ms	4409 ms	+4 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[sejas]

Approving but I'm wondering if we can go directly to stable.

Agreed! Let's aim to move it to stable to all users in next Studio release. Let's give it a bit more test these days and remove the beta flag too.