fix(promos): guard stored discount percent

[aiirvizionz]

Summary

• reuse the existing promo percent normalizer when reading stored promo rows
• keep checkout quotes and admin lists from surfacing NaN when stored promo data is malformed
• add regression coverage for numeric stored values and decimal-string fallback

Bug

quotePromo computed percentOff with Number(row.percent_off ?? 100) and clamped it directly. If a promo row is malformed or manually edited to a non-numeric value, Math.max(1, NaN) produces NaN, which makes finalUsd become NaN during checkout quoting instead of falling back to a safe discount value.

Validation

• node "node_modules/.pnpm/vitest@3.2.7_@types+node@22.20.0_tsx@4.23.0/node_modules/vitest/vitest.mjs" run apps/web/lib/promo-form.test.ts passed 4 tests