- ChequeService: add nextChequeNumberForPlan(planId) — returns next number
  scoped to the specific plan, not globally across all plans
- ChequeController::index(): pass nextChequeNumForPlan to view
- ChequeController::storeBatch(): load existing cheques before validation;
  check against existing numbers for duplicates; guard against exceeding
  requiredCount; coverage check uses existingTotal + batchTotal; only
  enforce full-coverage on the final batch
- cheques.php JS: generator uses REMAINING_COUNT (not amount-math) for count,
  NEXT_NUM_FOR_PLAN for sequence start — correctly continues from cheque 6
  if 5 already exist; preview shows "تكملة من #N" context note
- cheques.php UI: yellow info banner when existing cheques present, showing
  count, remaining, and starting cheque number
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>