Remove quick blink when starting Studio onboarding

[sejas]

Related issues

• Fixes STU-1091

Proposed Changes

• Avoid showing the zero sites layout when loading the onboarding.

Testing Instructions

• Remove your sites, log out, and set onboardingCompleted: false in your Studio config file.
• Run npm start or npm run make
• Observe that a empty dark screen while loading until it loads the onboarding.
• Press cmd+r multiple times
• Confirm the Sites layout with left main sidebar doesn't appear.

remove-blink-when-loading-onboarding.mp4

Pre-merge Checklist

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

[github-actions]

📊 Performance Test Results

Comparing 1ea436f vs trunk

site-editor

Metric	trunk	1ea436f	Diff	Change
load	10984.00 ms	11087.00 ms	+103.00 ms	🔴 0.9%

site-startup

Metric	trunk	1ea436f	Diff	Change
siteCreation	21574.00 ms	21798.00 ms	+224.00 ms	🔴 1.0%
siteStartup	9032.00 ms	10039.00 ms	+1007.00 ms	🔴 11.1%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change

[bcotrim]

LGTM 👍
I confirmed the issue on trunk and the fix here, nice catch!