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

spec/controllers/api/v1/accounts/contacts_controller_spec.rb

@@ -369,6 +369,50 @@ RSpec.describe 'Contacts API', type: :request do
         expect(response.body).to include(contact_special.identifier)
         expect(response.body).not_to include(contact_normal.identifier)
       end
+
+      it 'returns has_more as false when results fit in one page' do
+        get "/api/v1/accounts/#{account.id}/contacts/search",
+            params: { q: contact2.email },
+            headers: admin.create_new_auth_token,
+            as: :json
+
+        expect(response).to have_http_status(:success)
+        response_body = response.parsed_body
+        expect(response_body['meta']['has_more']).to be(false)
+        expect(response_body['meta']['count']).to eq(1)
+      end
+
+      it 'returns has_more as true when there are more results' do
+        # Create 16 contacts (more than RESULTS_PER_PAGE which is 15)
+        create_list(:contact, 16, account: account, name: 'searchable_contact')
+
+        get "/api/v1/accounts/#{account.id}/contacts/search",
+            params: { q: 'searchable_contact' },
+            headers: admin.create_new_auth_token,
+            as: :json
+
+        expect(response).to have_http_status(:success)
+        response_body = response.parsed_body
+        expect(response_body['meta']['has_more']).to be(true)
+        expect(response_body['meta']['count']).to eq(15)
+        expect(response_body['payload'].length).to eq(15)
+      end
+
+      it 'returns has_more as false on the last page' do
+        # Create 16 contacts
+        create_list(:contact, 16, account: account, name: 'searchable_contact')
+
+        get "/api/v1/accounts/#{account.id}/contacts/search",
+            params: { q: 'searchable_contact', page: 2 },
+            headers: admin.create_new_auth_token,
+            as: :json
+
+        expect(response).to have_http_status(:success)
+        response_body = response.parsed_body
+        expect(response_body['meta']['has_more']).to be(false)
+        expect(response_body['meta']['count']).to eq(1)
+        expect(response_body['payload'].length).to eq(1)
+      end
     end
   end