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/components/header.py

@@ -0,0 +1,47 @@
+"""Top header bar component matching 01_nhs_classic.html design."""
+from dash import html
+
+
+def make_header():
+    """Return the fixed top header with NHS branding and data freshness indicators."""
+    return html.Header(
+        className="top-header",
+        children=[
+            # Brand area (NHS logo + title) with angled clip-path
+            html.Div(
+                className="top-header__brand",
+                children=[
+                    html.Div("NHS", className="top-header__logo"),
+                    html.Div(
+                        html.Div("HCD Analysis", className="top-header__title"),
+                    ),
+                ],
+            ),
+            # Breadcrumb
+            html.Div(
+                className="top-header__breadcrumb",
+                children=[
+                    "Dashboard \u203A ",
+                    html.Strong("Pathway Analysis"),
+                ],
+            ),
+            # Right side: status dot + record count + last updated
+            html.Div(
+                className="top-header__right",
+                children=[
+                    html.Span(
+                        children=[
+                            html.Span(className="status-dot"),
+                            html.Span("...", id="header-record-count"),
+                        ],
+                    ),
+                    html.Span(
+                        children=[
+                            "Last updated: ",
+                            html.Span("...", id="header-last-updated"),
+                        ],
+                    ),
+                ],
+            ),
+        ],
+    )