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

run_dash.py

@@ -0,0 +1,5 @@
+"""Entry point for the Dash application."""
+from dash_app.app import app
+
+if __name__ == "__main__":
+    app.run(debug=True, port=8050)