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/store/modules/contacts/actions.js

@@ -45,14 +45,19 @@ export const handleContactOperationErrors = error => {
 };
 
 export const actions = {
-  search: async ({ commit }, { search, page, sortAttr, label }) => {
+  search: async (
+    { commit },
+    { search, page, sortAttr, label, append = false }
+  ) => {
     commit(types.SET_CONTACT_UI_FLAG, { isFetching: true });
     try {
       const {
         data: { payload, meta },
       } = await ContactAPI.search(search, page, sortAttr, label);
-      commit(types.CLEAR_CONTACTS);
-      commit(types.SET_CONTACTS, payload);
+      if (!append) {
+        commit(types.CLEAR_CONTACTS);
+      }
+      commit(append ? types.APPEND_CONTACTS : types.SET_CONTACTS, payload);
       commit(types.SET_CONTACT_META, meta);
       commit(types.SET_CONTACT_UI_FLAG, { isFetching: false });
     } catch (error) {