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/components-next/Contacts/ContactsLoadMore.vue

@@ -0,0 +1,28 @@
+<script setup>
+import { useI18n } from 'vue-i18n';
+import Button from 'dashboard/components-next/button/Button.vue';
+
+defineProps({
+  isLoading: {
+    type: Boolean,
+    default: false,
+  },
+});
+
+const emit = defineEmits(['loadMore']);
+
+const { t } = useI18n();
+</script>
+
+<template>
+  <div class="flex justify-center py-4">
+    <Button
+      :label="t('CONTACTS_LAYOUT.LOAD_MORE')"
+      :is-loading="isLoading"
+      variant="faded"
+      color="slate"
+      size="sm"
+      @click="emit('loadMore')"
+    />
+  </div>
+</template>