feat: data enrichment pipeline + UI mockup

- enrich.py merges Behind the Name (20k names, CC-BY-SA 4.0) and
  carltonnorthern/nicknames (1.1k, Apache-2.0) into boys/girls CSV
- 94% of top 500 boys names now have meanings and origin categories
- preview-mockup.html shows all 5 proposed screens for review

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

Author: Bo Bobson