fix: make numberFormat always return a string

[grunch]

Closes #855

numberFormat had an inconsistent return type (false | number | string), and its results are interpolated directly into user-facing strings (order descriptions, order titles, trading volume). Nothing breaks today because every caller validates the currency first, but that invariant was implicit — the first future caller that skips the getCurrency check would publish a title like "Selling false sats" to the order channel.

Changes

• numberFormat now has an explicit : string return type and falls back to String(number) in every non-formattable case (invalid ISO-4217 code, unknown currency, missing locale, NaN).
• Updated tests/util/index.spec.ts to the new contract (TDD: tests updated first and confirmed failing, then the fix), including a new case asserting the return is always a string.

All 19 existing call sites (bot/ordersActions.ts, bot/messages.ts) already treat the result as display text; none compares against false, so no behavior changes for valid inputs.

Re: the suggestion to enable @typescript-eslint/explicit-function-return-type — measured it: 381 warnings repo-wide today, so it's a separate effort; this PR adds the explicit return type to the function at hand. Follow-up discussion in #855.

Test plan

• Updated tests fail against old implementation (RED) and pass against new one (GREEN)
• Full suite: 151 passing, 0 failing
• npm run lint clean
• npx tsc full build clean

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📥 Commits

Reviewing files that changed from the base of the PR and between 0e87ea9 and 187582a.

📒 Files selected for processing (2)

• tests/util/index.spec.ts
• util/index.ts

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch fix/numberformat-always-string

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