Studio: Refactor `studio preview create` to adjust new site logic

[bcotrim]

Related issues

• Fixes STU-991

Proposed Changes

• studio preview create now requires the target folder to be an existing Studio site, failing with a helpful error if not
• Removed newSites concept from app and cli entirely

Testing Instructions

• npm start
• Create a Studio site and remove it without deleting files
• Close Studio
• node dist/cli/main.js preview --path <new site folder>
• Verify it fails.
• Verify studio preview create <existing-site-folder> works correctly for existing Studio sites

Pre-merge Checklist

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

[github-actions[bot]]

📊 Performance Test Results

Comparing 72ea508cecf17e12416f204758d5d64c9975a29d vs trunk

site-editor

Metric	trunk	72ea508cecf17e12416f204758d5d64c9975a29d	Diff	Change
load	9347.00 ms	11657.00 ms	+2310.00 ms	🔴 24.7%

site-startup

Metric	trunk	72ea508cecf17e12416f204758d5d64c9975a29d	Diff	Change
siteCreation	16049.00 ms	14082.00 ms	-1967.00 ms	🟢 -12.3%
siteStartup	7041.00 ms	6972.00 ms	-69.00 ms	🟢 -1.0%

---

Results are median values from multiple test runs.

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