feat: Add infinite scroll to contacts search page (#13376)

## Summary
- Add `has_more` to contacts search API response to enable infinite
scroll without expensive count queries
- Set `count` to the number of items in the current page instead of
total count
- Implement "Load more" button for contacts search results
- Keep existing contacts visible while loading additional pages

## Changes

### Backend
- Add `fetch_contacts_with_has_more` method that fetches N+1 records to
determine if more pages exist
- Return `has_more` in search endpoint meta response
- Set `count` to current page size instead of total count

### Frontend
- Add `APPEND_CONTACTS` mutation for appending contacts without clearing
existing ones
- Update search action to support `append` parameter
- Add `ContactsLoadMore` component with loading state
- Update `ContactsListLayout` to support infinite scroll mode
- Update `ContactsIndex` to use infinite scroll for search view

app/views/api/v1/accounts/contacts/search.json.jbuilder

@@ -1,6 +1,7 @@
 json.meta do
   json.count @contacts_count
   json.current_page @current_page
+  json.has_more @has_more
 end
 
 json.payload do