fix(creditos): show latest month balance instead of summing all months

Saldo insoluto is a balance snapshot, not cumulative. The creditos page was summing records across all filtered months, inflating the total. Now finds the latest fecha in the filtered set and computes KPIs and bar chart from only that month's records.

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com