Fix: CLI doesn't generate site thumbnails

[gcsecsey]

Related issues

• Fixes STU-1380

How AI was used in this PR

Claude was used to iterate on an initial plan, then to carry out the changes.

Proposed Changes

• Create a shared thumbnail capture function captureSiteThumbnail used by both the CLI events subscriber and the desktop UI
• Remove duplicate thumbnail logic from loadThemeDetails and delegate to captureSiteThumbnail
• Add deduplication logic to prevent concurrent BrowserWindow creation when multiple code paths capture the same site's thumbnail
• Update tests

Testing Instructions

1. Build CLI with npm run cli:build
2. Start the app with npm start
3. In a new terminal, create a site via CLI

node apps/cli/dist/cli/main.js site create --path ~/Studio/cli-thumb-test

4. In the desktop app, check that the site appears with a thumbnail
5. Create a new site on the desktop app and check that thumbnail generation still works as expected

trunk	this branch

Pre-merge Checklist

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

[bcotrim]

LGTM 👍
Tested the changes and worked as expected, nice improvement!

Added one comment, but it's a suggestion not a blocker.

[wpmobilebot]

📊 Performance Test Results

Comparing 4c19a34 vs trunk

app-size

Metric	trunk	4c19a34	Diff	Change
App Size (Mac)	1236.63 MB	1236.63 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	4c19a34	Diff	Change
load	1591 ms	1940 ms	+349 ms	🔴 21.9%

site-startup

Metric	trunk	4c19a34	Diff	Change
siteCreation	7072 ms	6088 ms	984 ms	🟢 -13.9%
siteStartup	3937 ms	3934 ms	3 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

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