fix: stop dropping digits when pad is combined with a locale

[spokodev]

When pad is combined with locale, the padding step infers the decimal separator by taking the last ./, in the already-localized string:

// src/helpers.js, applyNumberFormatting
const x = separator || (resultStr.slice(1).match(/[.,]/g) || []).pop() || PERIOD;
const tmp = resultStr.split(x);

For a value the locale renders with grouping separators and no decimal part, that last separator is a grouping separator, so split produces several parts and only the first two are used. Digit groups are dropped and padding is skipped, so the formatted string no longer represents the value:

filesize(1234500e24, { locale: "en", pad: true, round: 2 })
// "1,234 YB"  — "500" lost, no padding; expected "1,234,500.00 YB"
filesize(1000e24, { locale: "en", pad: true, round: 2 })
// "1,000 YB"  — expected "1,000.00 YB"

The fix lets the locale formatter emit the fixed number of fraction digits via minimumFractionDigits / maximumFractionDigits, so the localized string is never re-parsed. The manual string padding now runs only for the non-locale paths (plain / custom separator), where the string has a single decimal separator and no grouping. The documented non-locale example (filesize(1536, {round: 3, pad: true}) → 1.536 kB) and the existing locale tests are unchanged.

Full suite passes (171); added a regression test for the integer-with-grouping case.

(dist/ left unbuilt so the diff stays reviewable; happy to rebuild it if you prefer.)