fix: batch bulk delete to prevent silent failure on large selections

Replace the single-payload DELETE request with batched requests
(BULK_DELETE_BATCH_SIZE=100 items each) so large selections such as
50 K subscribers don't exceed implicit browser/server payload limits
that cause the request to be dropped without surfacing an error.

Changes:
- onDeleteSelected is now async and iterates in batches via await
- deleteProgress state drives a LinearProgress bar and disables the
  button while deletion is in progress (mirrors onCreate pattern)
- Console logs added at each batch for browser-side diagnostics
- ariaAttributes type widened to Record<string, string | number> to
  match react-window v2's aria prop signature
- Unused MUI Table/TableCell/TableHead/TableRow imports removed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ALIIQBAL786