numberFormat() returns false | number | string — latent 'Selling false sats' footgun

[grunch]

Context

Found while reviewing #852.

Problem

numberFormat in util/index.ts has an inconsistent return type:

const numberFormat = (code: string, number: number) => {
  if (!isIso4217(code)) return false;      // boolean
  if (!currencies[code]) return number;    // number
  ...
  if (!locale || isNaN(number)) return number; // number
  return numberToLocaleString.format(number);  // string
};

Its results are interpolated directly into user-facing strings (order descriptions, order titles via #852, trading volume). Today nothing breaks because every current caller validates the currency first (e.g. createOrder checks getCurrency(fiatCode) before buildDescription), but that invariant is implicit. The first future caller that skips the check publishes a title like "Selling false sats" to the order channel.

Suggested fix

Make numberFormat always return a string:

• String(number) as the fallback instead of returning the raw number;
• for the invalid-ISO-4217 case, either String(number) as well, or make the function throw / narrow the signature so TypeScript forces callers to handle it.

Low-risk change; all existing call sites already treat the result as display text.

[knocte]

Maybe it would be interesting to enable the ESLint rule @typescript-eslint/explicit-function-return-type moving forward.

[grunch]

Good idea @knocte. I measured it: enabling @typescript-eslint/explicit-function-return-type today produces 381 warnings repo-wide, so flipping it to error in one go isn't practical. A reasonable path: enable it as warn now (CI stays green, new code gets nudged), annotate incrementally, and promote to error once the count reaches zero. #858 adds the explicit : string to numberFormat as a first step. If maintainers agree I can open the warn-enablement PR.