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>

portfolio_app/figures/summary_cards.py

@@ -69,7 +69,8 @@ def create_metric_card_figure(
         height=120,
         margin={"l": 20, "r": 20, "t": 40, "b": 20},
         paper_bgcolor="rgba(0,0,0,0)",
-        font={"family": "Inter, sans-serif"},
+        plot_bgcolor="rgba(0,0,0,0)",
+        font={"family": "Inter, sans-serif", "color": "#c9c9c9"},
     )
 
     return fig