font-mono (monospace) renders optically larger than the surrounding
sans-serif at the same em size. Removing it from quantity, unit price,
taxable, GST% and total cells in both read-only and edit mode makes
all numbers visually consistent with the rest of the page.
formatCurrency reverted to Intl.NumberFormat with style:currency (the
original implementation); the spacing issue the user saw was font-related,
not symbol-related.