Interest/APY rounded consistently

Two calculation sites that both compute interest but use different rounding functions — one Math.round(), one Math.floor() — will produce different balances for identical inputs. The discrepancy shows up as a mismatch between the displayed APY and the actual accrued interest, leading to customer disputes and reconciliation failures. CWE-682 covers incorrect financial calculation; ISO 25010 maintainability requires that the same logical operation produce the same result regardless of call site.

Centralize interest rounding into a single exported function in src/lib/finance.ts and replace every direct Math.round / Math.floor / .toFixed call in financial logic with it.

// src/lib/finance.ts
import Decimal from 'decimal.js';

/** Canonical rounding for all interest / APY calculations. */
export function roundInterestCents(value: number): number {
  return new Decimal(value)
    .toDecimalPlaces(0, Decimal.ROUND_HALF_EVEN)
    .toNumber();
}

Search src/ for Math.round, Math.floor, and .toFixed within files that reference interest, apy, or rate, and replace each with roundInterestCents().