feat: add landing page at / with navigation links

Route GET / to a landing page with the Porchlight logo, tagline,
and card-style navigation links to My Account and Administration.

tests/test_app.py

@@ -28,6 +28,18 @@ async def test_app_has_repos_on_state(client: AsyncClient) -> None:
     assert isinstance(app.state.magic_link_repo, MagicLinkRepository)
 
 
+async def test_landing_page(client: AsyncClient) -> None:
+    response = await client.get("/")
+    assert response.status_code == 200
+    assert "text/html" in response.headers["content-type"]
+    body = response.text
+    assert "<h1>Porchlight</h1>" in body
+    assert "/manage/profile" in body
+    assert "/admin/users" in body
+    assert "My Account" in body
+    assert "Administration" in body
+
+
 async def test_dependency_functions() -> None:
     from unittest.mock import MagicMock