Feat: implement loan installment due date tracking

[Oluwatos94]

Closes #22

Description

Completes loan installment due-date tracking in the creditline contract. Adds an
is_on_time() helper to RepaymentInstallment and enforces that every installment
carries a concrete, future due date before a loan can be activated. (The due_date
and paid_at fields, paid_at captures on repayment, and timing-based reputation
updates were already present; this PR fills the remaining gaps.)

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changes Made

• types.rs: Add RepaymentInstallment::is_on_time() → returns paid && paid_at <= due_date
  (paying exactly on the due date counts as on time; unpaid installments are never on time).
• errors.rs: Add InvalidDueDate = 27 error variant.
• lib.rs: approve_loan now rejects loans with an empty schedule or any installment whose
  due_date is 0 (unset) or already in the past. Caller-supplied schedules are preserved —
  nothing is overwritten. The check runs after the existing status/auth checks, so those errors
  still take precedence.

Testing

• Tests pass locally — cargo test -p creditline-contract: 113 passed, 0 failed
• New tests added (if applicable)
  • test_installment_is_on_time — early / exact / late / unpaid cases
  • test_repay_installment_on_time_reflected_by_is_on_time
  • test_repay_installment_late_reflected_by_is_on_time
  • test_approve_loan_rejects_zero_due_date
  • test_approve_loan_rejects_past_due_date

Checklist

• Code follows project style guidelines
• Self-review completed
• Comments added for complex code
• Documentation updated — n/a (no user-facing docs affected)
• No new warnings generated — cargo clippy -p creditline-contract is clean

[Oluwatos94]

@EmeditWeb, This is ready for review. Thanks!