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/mutations.js

@@ -15,9 +15,24 @@ export const mutations = {
   },
 
   [types.SET_CONTACT_META]: ($state, data) => {
-    const { count, current_page: currentPage } = data;
+    const { count, current_page: currentPage, has_more: hasMore } = data;
     $state.meta.count = count;
     $state.meta.currentPage = currentPage;
+    if (hasMore !== undefined) {
+      $state.meta.hasMore = hasMore;
+    }
+  },
+
+  [types.APPEND_CONTACTS]: ($state, data) => {
+    data.forEach(contact => {
+      $state.records[contact.id] = {
+        ...($state.records[contact.id] || {}),
+        ...contact,
+      };
+      if (!$state.sortOrder.includes(contact.id)) {
+        $state.sortOrder.push(contact.id);
+      }
+    });
   },
 
   [types.SET_CONTACTS]: ($state, data) => {