feat: add pathway data loading callback bridging filters to chart-data (Task 3.2)

IMPLEMENTATION_PLAN.md

@@ -172,11 +172,11 @@ Drawer selection → update_drug_selection → app-state store → load_pathway_
 - **Checkpoint**: Page loads reference data, filter dropdowns update app-state store (verify via browser dev tools → dcc.Store)
 
 ### 3.2 Pathway data loading callback
-- [ ] Create `dash_app/callbacks/chart.py` (or add to filters.py):
+- [x] Create `dash_app/callbacks/chart.py` (or add to filters.py):
   - `load_pathway_data` callback: Input=`app-state` store, Output=`chart-data` store
   - Calls `queries.load_pathway_data(filter_id, chart_type, selected_drugs, selected_directorates)`
   - Runs on page load AND whenever `app-state` changes
-- **Checkpoint**: Changing date filter updates chart-data store with new pathway nodes
+- **Checkpoint**: Changing date filter updates chart-data store with new pathway nodes ✓
 
 ### 3.3 KPI update callback
 - [ ] Create `dash_app/callbacks/kpi.py`:

dash_app/callbacks/__init__.py

@@ -4,5 +4,7 @@
 def register_callbacks(app):
     """Register all Dash callbacks with the app instance."""
     from dash_app.callbacks.filters import register_filter_callbacks
+    from dash_app.callbacks.chart import register_chart_callbacks
 
     register_filter_callbacks(app)
+    register_chart_callbacks(app)

dash_app/callbacks/chart.py

@@ -0,0 +1,29 @@
+"""Callback for loading pathway data from SQLite into chart-data store."""
+from dash import Input, Output, callback, no_update
+
+
+def register_chart_callbacks(app):
+    """Register pathway data loading callback."""
+
+    @app.callback(
+        Output("chart-data", "data"),
+        Input("app-state", "data"),
+    )
+    def load_pathway_data(app_state):
+        """Load pathway nodes when app-state changes (filter or chart type)."""
+        if not app_state:
+            return no_update
+
+        from dash_app.data.queries import load_pathway_data as query_pathway_data
+
+        filter_id = app_state.get("date_filter_id", "all_6mo")
+        chart_type = app_state.get("chart_type", "directory")
+        selected_drugs = app_state.get("selected_drugs") or None
+        selected_directorates = app_state.get("selected_directorates") or None
+
+        return query_pathway_data(
+            filter_id=filter_id,
+            chart_type=chart_type,
+            selected_drugs=selected_drugs,
+            selected_directorates=selected_directorates,
+        )