Period format is "H4 (2024.12.25 - 2026.06.22)" — use regex to extract
the date before the closing paren instead of split+strip which fails on
the trailing ")".
Parse end date from Settings.period ("2024.01.01 - 2025.06.22") and use
it as reference for gap_days. Falls back to datetime.now() only if period
cannot be parsed. Rename output key gap_days_to_now → gap_days_to_end.