test: add unit tests for frontend bignumber and utilities helpers

[devin-ai-integration]

Summary

Coverage analysis: the repo had effectively no meaningful unit tests. Blockchain has only the default Hardhat test/Lock.ts (which references a non-existent Lock contract), backend has none, and frontend has only the default CRA App.test.tsx. Measuring the frontend src/helpers / src/constants modules confirmed 0% coverage across the board.

The two largest, least-covered pure-logic modules are frontend/src/helpers/bignumber.ts (204 lines) and frontend/src/helpers/utilities.ts (220 lines) — ideal for unit testing since they don't require network/web3 mocks. This PR adds Jest tests (run via the existing react-app-rewired test) for them.

Coverage for the two files after this PR:

File          | % Stmts | % Funcs | % Lines
bignumber.ts  |     100 |     100 |     100   (was 0)
utilities.ts  |   37.31 |      50 |   37.31   (was 0)

utilities.ts is partially covered because most of it is async network/API calls (apiGet*, formatTestTransaction); the tests cover the pure functions: isObject, sanitizeHex, convertStringToHex, convertNumberToString, getChainData (known chain, unknown-chain fallback, and Infura %API_KEY% substitution branch), and hashPersonalMessage (deterministic EIP-191 hash).

bignumber.ts tests cover all exported predicates, conversions, comparisons, arithmetic (incl. big-integer precision), raw-number conversion round-trips, and the formatting helpers.

Notes:

• recoverPublicKey was intentionally not tested — its secp256k1 path rejects jsdom's browserified Buffer polyfill (Expected private key to be an Uint8Array), so it isn't reliably unit-testable in the CRA jsdom environment.
• 46 tests total, all passing; tsc --noEmit is clean.

Test plan

cd frontend
npm i
CI=true npx react-app-rewired test src/helpers/bignumber.test.ts src/helpers/utilities.test.ts --watchAll=false

Link to Devin session: https://app.devin.ai/sessions/0fee9282ef36488383c821a634211ba4
Requested by: @walnutwaldo

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