feat(transfers): gender-based separation logic — males use work/grad/age25, females use marriage date

- Males: effective_date = min(max(work_date, graduation_date), date_turned_25) — unchanged
- Females: effective_date = marriage_date only (no work/graduation/age25 fields)
- Add data-gender attribute to child select options for JS detection
- Show male-specific fields (employment, graduation, work_date, date25) only for males
- Show marriage_date field only for females
- Females bypass age >= 25 filter in dropdown (eligible at any age via marriage)
- Server-side age validation skipped for female children
- Add marriage_date column to migration and model fillable
- Frontend dynamically switches between male/female form based on selected child's gender
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>