fix(auth): a cancel at Google lands where the browser is; the probe says why it refused

A refused session keeps its cookie, so the cancel branch, keyed on the cookie's account, sent a
signed-out browser to the in-app toast the login page never shows: it now asks whether the
session holds. The probe's log line named every refusal 'its account no longer holds'; it now
carries the reason. The pruning docstring says again that a renewal may land a request's time
after the revoke, and the test pins the day of margin. The test clock clamps in the zone asked for.

backend/tests/test_google_callback.py

@@ -59,6 +59,29 @@ def _sign_in(client, google, state: str | None = None, **userinfo):
     )
 
 
+@pytest.mark.parametrize("session", ["holds", "refused"])
+def test_a_cancel_at_google_lands_where_the_browser_is(client, google, make_user, session):
+    # Signed in, the cancel is a toast in the app; signed out, the login page's banner. A refused
+    # session keeps its cookie and its account id, and its browser is on the login page.
+    import base64
+    import json
+
+    from itsdangerous import TimestampSigner
+
+    make_user(email="cancel@example.com", password=_PW)
+    _password_login(client, "cancel@example.com")
+    if session == "refused":
+        signer = TimestampSigner(str(settings.secret_key))
+        payload = json.loads(base64.b64decode(signer.unsign(client.cookies["session"])))
+        payload.pop("sid")  # a cookie from before session ids
+        refused = signer.sign(base64.b64encode(json.dumps(payload).encode())).decode()
+        client.cookies.clear()
+        client.cookies.set("session", refused)
+    r = _sign_in(client, google, state="not-this-flow")
+    expected = "/?oauth=cancelled" if session == "holds" else "/?login=oauth_cancelled"
+    assert r.headers["location"] == expected
+
+
 def _rows(db, model, column, value):
     db.expire_all()
     return db.execute(select(model).where(column == value)).scalars().all()