Make the number of records loaded per group in grouped views configurable

Scope & Context

When a record table or list is grouped by a field, each group fetches a hardcoded 8 records and shows a "Load more" button below them (#8756). Self-hosted instances with a workflow that relies on grouped views have no way to see groups expanded by default, so users click "Load more" on every group every time they open the view.

Current behavior

useFindManyRecordIndexTableParams sets limit: 8 whenever a record group definition is active. The value is not configurable.

Expected behavior

The per-group page size should be configurable per instance, defaulting to 8 so existing instances are unaffected. A config variable (env or admin panel) is the natural place, since this is an instance-level operational preference rather than a per-view setting.

Proposed solution

Add a RECORD_GROUP_PAGE_SIZE config variable (default 8, ADVANCED_SETTINGS group), expose it on ClientConfig, and use it as the grouped limit in useFindManyRecordIndexTableParams. "Load more" keeps working and uses the same page size.

Implementation: #25662