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/pages/home.py

@@ -0,0 +1,14 @@
+"""Bio landing page."""
+
+import dash
+from dash import html
+
+dash.register_page(__name__, path="/", name="Home")
+
+layout = html.Div(
+    [
+        html.H1("Analytics Portfolio"),
+        html.P("Welcome to Leo's analytics portfolio."),
+        html.P("Dashboard coming soon."),
+    ]
+)