Implement RTK Query approach for fetching wpcom sites

[epeicher]

Related issues

• Related STU-1035
• Related STU-710

Before

CleanShot.2025-11-26.at.20.17.26-trimmed.mp4

After

CleanShot.2025-11-26.at.20.18.33.mp4

Proposed Changes

• Migrated from custom useFetchWpComSites hook to RTK Query with useGetWpComSitesQuery. The new query uses the userId to invalidate the cache
• Improved state management with automatic caching, background refetching, and request deduplication
• Reorganized sync utilities:
  • Moved sync-related utilities from src/hooks/use-fetch-wpcom-sites/ to src/modules/sync/lib/
  • Consolidated types into src/modules/sync/types.ts
  • Created shared sync-support.ts module for getSyncSupport and helper functions
• Updated all imports across 20+ files to reflect new structure
• Removed deprecated hook directory
• Enables by default the Streamline Onboarding feature flag

Testing Instructions

• Apply this branch and run npm start
• Use or create a site that does not have any connected sites
• Click on Publish site button and observe that the loading experience is fine (there are no multiple loading indicators)
• On the Sync tab, click on Publish site and check that the loading is minimal as the sites are cached
• Test also the Pull site button
• Delete or create a new site on WordPress.com
• Click on the Publish site button, check that the deleted or created site is displayed (there could be a delay)
• Log out and log in with a user with no sites
• Click on the Publish site button
• Check that the Find a perfect plan modal is displayed
• Check different use cases and confirm that the sites are correctly loaded

Pre-merge Checklist

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

[github-actions]

📊 Performance Test Results

Comparing 21ec897 vs trunk

site-editor

Metric	trunk	21ec897	Diff	Change
load	13313.00 ms	9449.00 ms	-3864.00 ms	🟢 -29.0%

site-startup

Metric	trunk	21ec897	Diff	Change
siteCreation	28598.00 ms	24404.00 ms	-4194.00 ms	🟢 -14.7%
siteStartup	9006.00 ms	8061.00 ms	-945.00 ms	🟢 -10.5%

---

Results are median values from multiple test runs.

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

[sejas]

Great work! Thanks for moving the wpcom sites request to RTK. Now it doesn't block the user every time we click on the buttons.
I just noticed that we display a loading state in the push site button every time we change tabs.

loading-push-site-button.mp4

[sejas]

Would be possible to refetch wpcom sites inside openModal action or call it at the same time?