fix: use exact decimal math and fix the displayed swap rate (Closes #159)

[waterWang]

Summary

Fixes the swap preview rate bug: it previously rendered the entire quoted output amount as 1 tokenIn = ..., instead of computing output ÷ input. Also replaces all floating-point Number arithmetic in the financial path with exact BigInt decimal math.

Closes #159

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Tests
• Chore / tooling

What Changed

New: lib/math.ts

• toBaseUnits(display, decimals) — parse human input into integer base units with BigInt only: no Number coercion, no scientific-notation / truncation surprises, excess decimals truncated safely.
• fromBaseUnits(base, decimals, maxDecimals) — exact base-unit → display formatting.
• computeRate(amountOut, amountIn, outDecimals, inDecimals) — rate = output ÷ input with the correct decimal scale in both directions (e.g. 10 USDC → 1 XLM = 10 USDC/XLM and reversed). Uses integer division with a scaled dividend, never floating point.
• applySlippage(amountOut, slippageBps) — integer basis-point minimum-output math, explicitly rounding down (favorable to safety).
• MathError — structured error with a machine-readable tag (invalid / overflow / division_by_zero).
• roundTripDisplay — property-test helper verifying display → base → display round trips.

lib/format.ts

• stroopsToXlm / rawToUsdc now use BigInt division through formatDecimals instead of Number / — exact for every input, including values beyond Number.MAX_SAFE_INTEGER.
• xlmToStroops / usdcToRaw delegate to toBaseUnits.

components/SwapPreview.tsx

• Rate row now shows computeRate(amountOut, amountIn, ...) instead of the raw output amount — the actual per-1-unit exchange rate.

app/swap/page.tsx

• minAmountOut now uses applySlippage() (documented rounding direction) instead of inline BigInt.

jest.setup.ts + jest.config.js

• Sets IS_REACT_ACT_ENVIRONMENT and NODE_ENV=test so React 19 + jsdom component tests stop crashing with React.act is not a function.

How Was It Tested?

• New __tests__/lib/math.test.ts: 22 cases covering base-unit conversion, exact large/small amounts, rate both directions, boundary ratios, slippage rounding direction, invalid input, and round-trip property checks.
• __tests__/lib/format.test.ts: unchanged expectations all pass (BigInt refactor is behavior-preserving).
• Full suite: 55 tests, 6 suites, all passing (NODE_ENV=development npx jest).
• npx tsc --noEmit — clean.
• npm run lint (eslint) — no errors.

Checklist

• My code follows the style guidelines in CONTRIBUTING.md
• I have rebased onto the latest main (no merge commits)
• I have added tests for my changes
• All existing tests pass locally
• I have linked the issue above (Closes #159)
• I have updated documentation if needed