feat: add two-view architecture with sidebar navigation (Task 10.2)

- Add active_view and selected_comparison_directorate to app-state
- Sidebar: rename to Patient Pathways + add Trust Comparison nav item
- View container pattern: two view divs toggled by active_view
- Navigation callback: sidebar clicks switch views + update active state
- Trust Comparison placeholder landing page with tc-landing structure

dash_app/callbacks/navigation.py

@@ -0,0 +1,29 @@
+"""Callbacks for view switching between Patient Pathways and Trust Comparison."""
+from dash import Input, Output
+
+
+def register_navigation_callbacks(app):
+    """Register view switching callbacks."""
+
+    @app.callback(
+        Output("patient-pathways-view", "style"),
+        Output("trust-comparison-view", "style"),
+        Output("nav-patient-pathways", "className"),
+        Output("nav-trust-comparison", "className"),
+        Input("app-state", "data"),
+    )
+    def switch_view(app_state):
+        """Show/hide views and update sidebar active state based on active_view."""
+        if not app_state:
+            return {}, {"display": "none"}, "sidebar__item sidebar__item--active", "sidebar__item"
+
+        view = app_state.get("active_view", "patient-pathways")
+        show = {}
+        hide = {"display": "none"}
+        active_cls = "sidebar__item sidebar__item--active"
+        inactive_cls = "sidebar__item"
+
+        if view == "patient-pathways":
+            return show, hide, active_cls, inactive_cls
+        else:
+            return hide, show, inactive_cls, active_cls