feat(investment): deal time, interest cycle and charges on Purchase Entry

As per Jitendra Sir's review:
- Deal Time is now a mandatory field next to Deal Date.
- New Interest Cycle section: the two coupon days, last and next coupon,
  next coupon amount and days to it.
- Stamp duty, brokerage and other charges sit on the main form. They are
  expenses on their own GLs (500601/500602/500603), so they are added to
  the settlement amount but no longer to the book value, and each shows
  as its own debit line in the accounting preview.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/investment/investment.service.ts

@@ -97,6 +97,7 @@ export interface InvestmentPurchaseDto {
   balanceUnits: number;
   brokerage?: number;
   stampDuty?: number;
+  otherCharges?: number;
   bookValue: number;
   premiumDiscount?: number;
   intRate?: number;