refactor(retroactive-wizard): group subscriptions by year instead of per-person rows

Step 6 now shows year cards where each financial year contains all family
members (member + spouses + children + temps) with a single shared status
dropdown and payment date. Replaces the flat per-person-per-year table.
Backend format unchanged — same hidden input naming for full compatibility.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>