Add loading and empty placeholders to the list of sites

[epeicher]

Related issues

• Closes STU-1080

Proposed Changes

• Extracts the content of the SyncSitesSelector modal to an external component and uses it as part of the pull-remote-site component so both uses the same Loading... and empty search logic
• Adds a new Gutenberg color to use the same style than in the Gutenberg modal

Before

CleanShot.2025-12-04.at.13.11.46.mp4

After

CleanShot.2025-12-04.at.13.10.00.mp4

Testing Instructions

• Apply this branch and run npm start
• Click on Add site and select Pull an existing site
• Verify you can see the Loading... while the sites are loaded, if not you can hit Cmd+R in dev to refresh the data and repeat the step
• Filter by a non existing site on the search
• Verify that you can see the message about No sites found for...

Regression testing

• Navigate to the Sync tab and conenct a site
• Check that there is not any regression in the list of sites displayed

Pre-merge Checklist

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

[github-actions]

📊 Performance Test Results

Comparing 4e86e0a vs trunk

site-editor

Metric	trunk	4e86e0a	Diff	Change
load	9258.00 ms	9633.00 ms	+375.00 ms	🔴 4.1%

site-startup

Metric	trunk	4e86e0a	Diff	Change
siteCreation	20251.00 ms	18435.00 ms	-1816.00 ms	🟢 -9.0%
siteStartup	8972.00 ms	10065.00 ms	+1093.00 ms	🔴 12.2%

---

Results are median values from multiple test runs.

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

[bcotrim]

As a new user without any sites on WordPress.com, I see an empty screen. Similarly to using a filter without results, I think we could show a more user friendly message to the user.

Code LGTM 👍

[sejas]

As a new user without any sites on WordPress.com, I see an empty screen. Similarly to using a filter without results, I think we could show a more user friendly message to the user.

We are tackling that under STU-1085

[sejas]

Works as expected. I confirm the loading state appears. Now the list of sites is more consistent between the Pull an existing site and the they sync modal.

I added a message for new users without any site as suggested by @bcotrim 7681ea0