feat: Add floating sidebar navigation and dark theme support

- Add floating pill-shaped sidebar with navigation icons
- Implement dark/light theme toggle with localStorage persistence
- Update all figure factories for transparent backgrounds
- Use carto-darkmatter map style for choropleths
- Add methodology link button to Toronto dashboard header
- Add back to dashboard button on methodology page
- Remove social links from home page (now in sidebar)
- Update CLAUDE.md to Sprint 7

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

CLAUDE.md

@@ -6,7 +6,7 @@ Working context for Claude Code on the Analytics Portfolio project.
 
 ## Project Status
 
-**Current Sprint**: 1 (Project Bootstrap)
+**Current Sprint**: 7 (Navigation & Theme Modernization)
 **Phase**: 1 - Toronto Housing Dashboard
 **Branch**: `development` (feature branches merge here)
 
@@ -254,4 +254,4 @@ All scripts in `scripts/`:
 
 ---
 
-*Last Updated: Sprint 1*
+*Last Updated: Sprint 7*