SPA tests: extract parseMoney helper + money round-trip tests #108

Follow-up to #105 / PR #106. The display→minor-units parse is duplicated inline in 3 route sites with no helper and no test, and is float-fragile.

Refactor

• Add parseMoney(text: string, minorUnit: number): number next to formatMoney in web/src/api/cluckwork.ts — the inverse of formatMoney: Math.round(parseFloat(text) * 10 ** minorUnit), with defined handling for empty/NaN input.
• Replace the inline copies: routes/InventoryPage.tsx:160, routes/SalesPage.tsx:199, routes/SalesPage.tsx:212.

Test

• parseMoney — 2/0/3-decimal currencies, rounding of float artifacts (e.g. 10.10 @ minorUnit 2 → 1010, not 1009), negatives, empty/NaN → defined result.
• Round-trip: parseMoney(formatMoney(n, c, u).split(' ')[0], u) === n across a sample.

Part of #105, epic #14.