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/choropleth.py

@@ -39,6 +39,10 @@ def create_choropleth_figure(
     if center is None:
         center = {"lat": 43.7, "lon": -79.4}
 
+    # Use dark-mode friendly map style by default
+    if map_style == "carto-positron":
+        map_style = "carto-darkmatter"
+
     # If no geojson provided, create a placeholder map
     if geojson is None or not data:
         fig = go.Figure(go.Scattermapbox())
@@ -51,6 +55,9 @@ def create_choropleth_figure(
             margin={"l": 0, "r": 0, "t": 40, "b": 0},
             title=title or "Toronto Housing Map",
             height=500,
+            paper_bgcolor="rgba(0,0,0,0)",
+            plot_bgcolor="rgba(0,0,0,0)",
+            font_color="#c9c9c9",
         )
         fig.add_annotation(
             text="No geometry data available. Complete QGIS digitization to enable map.",
@@ -59,7 +66,7 @@ def create_choropleth_figure(
             x=0.5,
             y=0.5,
             showarrow=False,
-            font={"size": 14, "color": "gray"},
+            font={"size": 14, "color": "#888888"},
         )
         return fig
 
@@ -68,6 +75,11 @@ def create_choropleth_figure(
 
     df = pd.DataFrame(data)
 
+    # Use dark-mode friendly map style
+    effective_map_style = (
+        "carto-darkmatter" if map_style == "carto-positron" else map_style
+    )
+
     fig = px.choropleth_mapbox(
         df,
         geojson=geojson,
@@ -76,7 +88,7 @@ def create_choropleth_figure(
         color=color_column,
         color_continuous_scale=color_scale,
         hover_data=hover_data,
-        mapbox_style=map_style,
+        mapbox_style=effective_map_style,
         center=center,
         zoom=zoom,
         opacity=0.7,
@@ -86,10 +98,17 @@ def create_choropleth_figure(
         margin={"l": 0, "r": 0, "t": 40, "b": 0},
         title=title,
         height=500,
+        paper_bgcolor="rgba(0,0,0,0)",
+        plot_bgcolor="rgba(0,0,0,0)",
+        font_color="#c9c9c9",
         coloraxis_colorbar={
-            "title": color_column.replace("_", " ").title(),
+            "title": {
+                "text": color_column.replace("_", " ").title(),
+                "font": {"color": "#c9c9c9"},
+            },
             "thickness": 15,
             "len": 0.7,
+            "tickfont": {"color": "#c9c9c9"},
         },
     )