Implement RTK Query approach for connectedSites

[fredrikekelund]

Related issues

• Related to Fix connected sites when logging in #2083 (comment)

Proposed Changes

In #2083, we implemented a manual refetch of the list of connected sites when users log in to Studio. This prompted me to look into refactoring the state tree to use RTK Query instead, because with RTK Query, queries are automatically re-evaluated whenever a dependency changes (e.g., the currently logged-in user).

This PR does the following:

• Adds src/stores/sync/connected-sites-api.ts with an RTK Query API
• Removes src/stores/sync/connected-sites-hooks.ts and src/stores/sync/connected-sites-slice.ts
• Updates all the consumers of those files to use the new approach instead
• The modal open state and modal mode are now stored as component state in src/modules/sync/index.tsx. We don't have to go this way—we could keep it in the state tree if needed—but in my light testing, this approach worked fine.

Testing Instructions

TBD

Pre-merge Checklist

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