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/callbacks/__init__.py

@@ -0,0 +1,5 @@
+"""Application-level callbacks for the portfolio app."""
+
+from . import theme
+
+__all__ = ["theme"]

portfolio_app/callbacks/theme.py

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+"""Theme toggle callbacks using clientside JavaScript."""
+
+from dash import Input, Output, State, clientside_callback
+
+# Toggle theme on button click
+# Stores new theme value and updates the DOM attribute
+clientside_callback(
+    """
+    function(n_clicks, currentTheme) {
+        if (n_clicks === undefined || n_clicks === null) {
+            return window.dash_clientside.no_update;
+        }
+        const newTheme = currentTheme === 'dark' ? 'light' : 'dark';
+        document.documentElement.setAttribute('data-mantine-color-scheme', newTheme);
+        return newTheme;
+    }
+    """,
+    Output("theme-store", "data"),
+    Input("theme-toggle", "n_clicks"),
+    State("theme-store", "data"),
+    prevent_initial_call=True,
+)
+
+# Initialize theme from localStorage on page load
+# Uses a dummy output since we only need the side effect of setting the DOM attribute
+clientside_callback(
+    """
+    function(theme) {
+        if (theme) {
+            document.documentElement.setAttribute('data-mantine-color-scheme', theme);
+        }
+        return theme;
+    }
+    """,
+    Output("theme-init-dummy", "data"),
+    Input("theme-store", "data"),
+    prevent_initial_call=False,
+)