feat: add Porchlight branding with logo, favicon, and redesigned CSS theme

Rebrand from FastAPI OIDC OP to Porchlight with warm amber/gold identity:
- Add doorway-with-light SVG logo and 32x32 PNG favicon
- Rewrite style.css with full design system (color tokens, spacing scale,
  typography scale, section cards, button variants, dark mode)
- Update base template with site header, logo, and favicon
- Update all page titles and FastAPI app title to Porchlight

tests/test_app.py

@@ -12,7 +12,7 @@ async def test_app_has_title(client: AsyncClient) -> None:
     response = await client.get("/openapi.json")
     assert response.status_code == 200
     data = response.json()
-    assert data["info"]["title"] == "FastAPI OIDC OP"
+    assert data["info"]["title"] == "Porchlight"
 
 
 async def test_app_has_repos_on_state(client: AsyncClient) -> None: