Fix mansion tax map to show correct % of local properties affected

- Fixed calculation: use total stock (11,481) × share_pct / dwellings
  instead of annual sales / dwellings
- Edinburgh Southern now correctly shows ~2.9% (was 0.1%)
- Removed our £18.5m revenue estimate from descriptions
- Simplified methodology to focus on affected properties, not revenue
- Updated color scale for new range (0-3% instead of 0-0.1%)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MaxGhenis