Add shared navigation to every page

The pages were only reachable by typing URLs, and the digest had no navigation
at all. A single _nav.html partial now appears on all four, marking the current
page, so Triage / Digest / Tune / Config are one click apart.

Parameterised test asserts every page links to the others and marks itself
active, so a page added later cannot ship stranded.

tests/test_delivery.py

@@ -232,3 +232,14 @@ def test_labelling_is_disabled_when_no_secret_is_configured(tmp_path, monkeypatc
     from news import web
     web._VECTORS = None
     assert "labelling is disabled" in TestClient(web.app).get("/").text
+
+
+@pytest.mark.parametrize("path,active", [
+    ("/", "Triage"), ("/tune", "Tune"), ("/config", "Config"), ("/digest", "Digest")])
+def test_every_page_carries_the_nav(client, path, active):
+    """Navigation must not require knowing the URLs."""
+    c, _ = client
+    html = c.get(path).text
+    for target in ("/tune", "/config", "/digest"):
+        assert f'href="{target}"' in html, f"{path} is missing a link to {target}"
+    assert f'class="here">{active}<' in html, f"{path} does not mark itself active"