- genPreview: compute lastAmt = remaining - amt*(needed-1) to show exact
  total; display grand total (existing + new) when cheques already exist;
  highlight preview red if lastAmt would be ≤ 0
- generateCheques: guard against per-cheque amount too large (lastAmt ≤ 0)
  with clear Arabic error showing max allowed per-cheque value; last cheque
  always = remaining - amt*(needed-1) so existing+new always = PLAN_TOTAL
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>