feat: add app foundation (config.py, app.py, home page)

- config.py: Pydantic BaseSettings for env loading
- app.py: Dash factory with Pages routing
- pages/home.py: Placeholder landing page

Closes #7

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

portfolio_app/app.py

@@ -0,0 +1,37 @@
+"""Dash application factory with Pages routing."""
+
+import dash
+from dash import html
+
+from .config import get_settings
+
+
+def create_app() -> dash.Dash:
+    """Create and configure the Dash application."""
+    settings = get_settings()
+
+    app = dash.Dash(
+        __name__,
+        use_pages=True,
+        suppress_callback_exceptions=True,
+        title="Analytics Portfolio",
+    )
+
+    app.layout = html.Div(
+        [
+            dash.page_container,
+        ]
+    )
+
+    return app
+
+
+def main() -> None:
+    """Run the development server."""
+    settings = get_settings()
+    app = create_app()
+    app.run(debug=settings.dash_debug, host="0.0.0.0", port=8050)
+
+
+if __name__ == "__main__":
+    main()