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/javascript/dashboard/i18n/locale/en/contact.json

@@ -574,7 +574,8 @@
       "SEARCH_EMPTY_STATE_TITLE": "No contacts matches your search 🔍",
       "LIST_EMPTY_STATE_TITLE": "No contacts available in this view 📋",
       "ACTIVE_EMPTY_STATE_TITLE": "No contacts are active at the moment 🌙"
-    }
+    },
+    "LOAD_MORE": "Load more"
   },
   "CONTACTS_BULK_ACTIONS": {
     "ASSIGN_LABELS": "Assign Labels",