style: make Patient Pathways chart fill available viewport height (Task 11.3)

- Changed .main from min-height to height with overflow-y auto
- Added flex chain: #view-container → #patient-pathways-view → .chart-card
  → dcc.Loading wrapper → #chart-container → #pathway-chart
- All flex parents get min-height: 0 for proper flex shrinking
- dcc.Graph set to responsive=True with flex: 1 and minHeight: 0
- Trust Comparison view scrolls naturally via .main overflow-y auto

dash_app/assets/nhs.css

@@ -187,8 +187,17 @@ body {
     margin-left: var(--sidebar-w);
     margin-top: var(--header-total-h);
     padding: 24px;
-    min-height: calc(100vh - var(--header-total-h));
+    height: calc(100vh - var(--header-total-h));
     display: flex; flex-direction: column; gap: 20px;
+    overflow-y: auto;
+}
+
+/* View containers — flex chain for chart to fill height */
+#view-container {
+    flex: 1; display: flex; flex-direction: column; min-height: 0;
+}
+#patient-pathways-view {
+    flex: 1; display: flex; flex-direction: column; min-height: 0; gap: 20px;
 }
 
 /* ── Patient Pathways Filter Strip ── */
@@ -250,6 +259,18 @@ body {
     border: 1px solid var(--nhs-pale-grey);
     flex: 1;
     display: flex; flex-direction: column;
+    min-height: 0;
+}
+/* Flex chain through dcc.Loading wrapper → chart container → dcc.Graph */
+#chart-container {
+    flex: 1; display: flex; flex-direction: column; min-height: 0;
+}
+#pathway-chart {
+    flex: 1; min-height: 0;
+}
+.chart-card > .dash-loading-callback,
+.chart-card > .dash-loading-callback > div {
+    flex: 1; display: flex; flex-direction: column; min-height: 0;
 }
 .chart-card__header {
     padding: 16px 20px;

dash_app/components/chart_card.py

@@ -81,7 +81,8 @@ def make_chart_card():
                         children=[
                             dcc.Graph(
                                 id="pathway-chart",
-                                style={"minHeight": "500px", "flex": "1"},
+                                style={"flex": "1", "minHeight": "0"},
+                                responsive=True,
                                 config={
                                     "displayModeBar": True,
                                     "displaylogo": False,