feat: create dash_app skeleton with nhs.css and MantineProvider (Phase 0)

- dash_app/ directory structure: app.py, assets/, data/, components/, callbacks/, utils/
- run_dash.py entry point at project root
- Added dash>=2.14.0 and dash-mantine-components>=0.14.0 to pyproject.toml
- app.py: Dash app with MantineProvider wrapper and 3 dcc.Store components
- nhs.css: extracted from 01_nhs_classic.html (sans mock icicle CSS)
- Validated: app starts cleanly at localhost:8050

dash_app/app.py

@@ -0,0 +1,36 @@
+"""Dash application entry point with layout root and state stores."""
+from dash import Dash, html, dcc
+import dash_mantine_components as dmc
+
+app = Dash(
+    __name__,
+    suppress_callback_exceptions=True,
+)
+
+app.layout = dmc.MantineProvider(
+    children=[
+        # State stores
+        dcc.Store(id="app-state", storage_type="session", data={
+            "chart_type": "directory",
+            "initiated": "all",
+            "last_seen": "6mo",
+            "date_filter_id": "all_6mo",
+            "selected_drugs": [],
+            "selected_directorates": [],
+        }),
+        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(
+            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."),
+            ],
+        ),
+    ],
+)
+
+server = app.server