feat: add header and sidebar components for Dash layout (Task 2.1)

- header.py: NHS branded top bar with logo, title, breadcrumb,
  data freshness indicators (record count + last updated with IDs
  for callback updates)
- sidebar.py: Navigation with 7 items across Analysis/Reports
  sections, SVG icons via data URI, Drug Selection and Indications
  items have IDs for drawer open callbacks (Phase 4)
- app.py: Assembles header + sidebar + main content placeholder
- nhs.css: Added .sidebar__icon rule for img-based SVG icons

dash_app/app.py

@@ -2,6 +2,9 @@
 from dash import Dash, html, dcc
 import dash_mantine_components as dmc
 
+from dash_app.components.header import make_header
+from dash_app.components.sidebar import make_sidebar
+
 app = Dash(
     __name__,
     suppress_callback_exceptions=True,
@@ -21,13 +24,17 @@ app.layout = dmc.MantineProvider(
         dcc.Store(id="chart-data", storage_type="memory"),
         dcc.Store(id="reference-data", storage_type="session"),
 
-        # Placeholder layout — will be replaced by assembled components
-        html.Div(
+        # Page structure
+        make_header(),
+        make_sidebar(),
+        html.Main(
             className="main",
-            style={"marginLeft": "0", "marginTop": "0"},
             children=[
                 html.H1("HCD Analysis", style={"color": "#003087"}),
-                html.P("Dash application scaffolding complete. Components will be added in subsequent phases."),
+                html.P(
+                    "Layout scaffolding with header and sidebar. "
+                    "KPIs, filter bar, and chart card will be added in Task 2.2."
+                ),
             ],
         ),
     ],