fix(loans): offer Pay EMI on the earliest unpaid installment, inside its pay window

[parameshjava]

No migration. Two rules for when Pay EMI appears on /admin/loans/[loan_number].

Rule 1 — earliest unpaid installment only

Two buttons showed on a loan whose schedule had just been moved back to its correct month. Both were correct under the old rule, but offering them together let an admin settle #2 while #1 was still outstanding, and payEmi has no ordering check of its own. A paid row stranded between unpaid ones is a state several things handle badly:

• the schedule reads out of order
• re-pricing has to skip around it (one of the confirmed findings on that path)
• the date-shift repair refuses outright once anything is settled, so the loan can no longer be corrected

Rule 2 — inside the pay window

Payable used to mean "the 1st of the accrual month has passed", which opened the button as much as 40 days before the due date. Settling that early does the member no good: payEmi charges exactly what the installment says, so it moves cash without touching the interest.

₹20,000 spent as…	Interest saved
EMIs paid early	₹0
prepayment, reduce EMI	₹740
prepayment, reduce tenure	₹1,265

(the ₹1,00,000 / 8% / 12-month case, two installments paid)

An installment is now collectable from 15 days before it falls due, or the 1st of the month it falls due in — whichever comes first. The 15-day window is the rule; the month clause keeps an installment dated late in a month collectable from the start of that month rather than only in its last fortnight.

Worked through, for a schedule due on the 10th:

Today	Next unpaid	Offered?	Why
20 Jul	10 Aug	no	21 days off
26 Jul	10 Aug	yes	window opens
7 Aug	10 Aug	yes	current month
20 Aug	10 Sep	no	21 days off
26 Aug	10 Sep	yes	window opens

Arrears are unaffected. Anything past its due date is trivially inside the window, so an overdue installment stays collectable however old — the window bounds how far ahead an EMI can be settled, not how far behind. Catching up on two missed months still works: settle the earliest and the next one's button appears immediately.

Where the rule lives

Out of an inline filter in page.tsx and into payableInstallmentIds / payWindowOpensOn in emi-due.ts, beside the other EMI-due tallies — one definition of "payable", and testable.

Testing

emi-payable.test.ts — 14 cases: the window opening 15 days out and at the month start when that comes first, month and year boundary rolls, nothing offered beyond the window, offered from the opening day, only the earliest even when a later one is inside its own window, advancing once the earliest is paid, arrears collectable a year late, part-paid offered, paid/waived skipped when picking the earliest, nothing when all settled, picked by due date rather than row order, and an empty schedule.

npm test 352 passing · npm run lint clean (one pre-existing warning) · npm run build passes.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
aits-fcf-tracker	Ready	Preview	Aug 7, 2026 2:24pm