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/pages/toronto/dashboard.py

@@ -3,6 +3,7 @@
 import dash
 import dash_mantine_components as dmc
 from dash import dcc, html
+from dash_iconify import DashIconify
 
 from portfolio_app.components import (
     create_map_controls,
@@ -76,6 +77,17 @@ def create_header() -> dmc.Group:
             ),
             dmc.Group(
                 [
+                    dcc.Link(
+                        dmc.Button(
+                            "Methodology",
+                            leftSection=DashIconify(
+                                icon="tabler:info-circle", width=18
+                            ),
+                            variant="subtle",
+                            color="gray",
+                        ),
+                        href="/toronto/methodology",
+                    ),
                     create_year_selector(
                         id_prefix="toronto",
                         min_year=2020,

portfolio_app/pages/toronto/methodology.py

@@ -2,7 +2,8 @@
 
 import dash
 import dash_mantine_components as dmc
-from dash import html
+from dash import dcc, html
+from dash_iconify import DashIconify
 
 dash.register_page(
     __name__,
@@ -18,8 +19,18 @@ def layout() -> dmc.Container:
         size="md",
         py="xl",
         children=[
+            # Back to Dashboard button
+            dcc.Link(
+                dmc.Button(
+                    "Back to Dashboard",
+                    leftSection=DashIconify(icon="tabler:arrow-left", width=18),
+                    variant="subtle",
+                    color="gray",
+                ),
+                href="/toronto",
+            ),
             # Header
-            dmc.Title("Methodology", order=1, mb="lg"),
+            dmc.Title("Methodology", order=1, mb="lg", mt="md"),
             dmc.Text(
                 "This page documents the data sources, processing methodology, "
                 "and known limitations of the Toronto Housing Dashboard.",