feat: Trust Comparison landing page + directorate selector (Task 10.7)

- Add get_directorate_summary() query for per-directorate patient/drug counts
- Create trust_comparison.py with landing grid and 6-chart dashboard layout
- Wire directorate card clicks and back button through app-state callbacks
- Add TC landing and dashboard CSS per Phase 10 design spec
- Placeholder charts for 6 dashboard graphs (filled in Task 10.8)
- Chart type toggle clears selected directorate when switching modes

dash_app/callbacks/trust_comparison.py

@@ -0,0 +1,131 @@
+"""Callbacks for Trust Comparison landing page and dashboard navigation."""
+from dash import html, Input, Output, State, ctx, no_update
+import plotly.graph_objects as go
+
+
+def register_trust_comparison_callbacks(app):
+    """Register Trust Comparison view callbacks."""
+
+    @app.callback(
+        Output("tc-landing-grid", "children"),
+        Output("tc-landing-grid", "className"),
+        Output("tc-landing-desc", "children"),
+        Input("app-state", "data"),
+    )
+    def populate_landing_grid(app_state):
+        """Populate the landing page grid with directorate/indication cards."""
+        if not app_state:
+            return [], "tc-landing__grid", "Select a directorate to compare drug usage across trusts."
+
+        from dash_app.data.queries import get_directorate_summary
+
+        chart_type = app_state.get("chart_type", "directory")
+        date_filter_id = app_state.get("date_filter_id", "all_6mo")
+
+        summaries = get_directorate_summary(date_filter_id, chart_type)
+
+        # Build card buttons
+        cards = []
+        for item in summaries:
+            name = item["name"]
+            patients = item["patients"]
+            drugs = item["drugs"]
+            cards.append(
+                html.Button(
+                    className="tc-card",
+                    id={"type": "tc-selector", "index": name},
+                    n_clicks=0,
+                    children=[
+                        html.Div(name, className="tc-card__name"),
+                        html.Div(
+                            className="tc-card__stats",
+                            children=[
+                                html.Span(
+                                    f"{patients:,} patients",
+                                    className="tc-card__stat",
+                                ),
+                                html.Span("\u00b7", className="tc-card__dot"),
+                                html.Span(
+                                    f"{drugs} drugs",
+                                    className="tc-card__stat",
+                                ),
+                            ],
+                        ),
+                    ],
+                )
+            )
+
+        # Grid class: wider for indication mode (more items)
+        grid_cls = "tc-landing__grid"
+        if chart_type == "indication":
+            grid_cls += " tc-landing__grid--wide"
+
+        # Description text adapts to chart type
+        if chart_type == "indication":
+            desc = "Select an indication to compare drug usage across trusts."
+        else:
+            desc = "Select a directorate to compare drug usage across trusts."
+
+        return cards, grid_cls, desc
+
+    @app.callback(
+        Output("trust-comparison-landing", "style"),
+        Output("trust-comparison-dashboard", "style"),
+        Output("tc-dashboard-title", "children"),
+        Input("app-state", "data"),
+    )
+    def toggle_tc_subviews(app_state):
+        """Toggle between landing page and 6-chart dashboard."""
+        if not app_state:
+            return {}, {"display": "none"}, ""
+
+        selected = app_state.get("selected_comparison_directorate")
+        show = {}
+        hide = {"display": "none"}
+
+        if selected:
+            chart_type = app_state.get("chart_type", "directory")
+            label = "Indication" if chart_type == "indication" else "Directorate"
+            title = f"{selected} \u2014 Trust Comparison"
+            return hide, show, title
+        else:
+            return show, hide, ""
+
+    # Dashboard chart rendering will be added in Task 10.8.
+    # For now, register empty figure placeholders for the 6 chart IDs
+    # so the dcc.Graph components don't error on load.
+    _tc_chart_ids = [
+        "tc-chart-market-share",
+        "tc-chart-cost-waterfall",
+        "tc-chart-dosing",
+        "tc-chart-heatmap",
+        "tc-chart-duration",
+        "tc-chart-cost-effectiveness",
+    ]
+
+    for chart_id in _tc_chart_ids:
+        @app.callback(
+            Output(chart_id, "figure"),
+            Input("app-state", "data"),
+            prevent_initial_call=True,
+        )
+        def _placeholder_chart(app_state, _cid=chart_id):
+            """Placeholder — returns empty figure until Task 10.8 implements real charts."""
+            selected = (app_state or {}).get("selected_comparison_directorate")
+            if not selected:
+                return no_update
+            fig = go.Figure()
+            fig.update_layout(
+                template="plotly_white",
+                margin=dict(l=20, r=20, t=30, b=20),
+                height=300,
+                annotations=[
+                    dict(
+                        text="Chart will be implemented in Task 10.8",
+                        xref="paper", yref="paper",
+                        x=0.5, y=0.5, showarrow=False,
+                        font=dict(size=14, color="#999"),
+                    )
+                ],
+            )
+            return fig