Restore UA-aware auth header advertisement

Revert accidental removal of WWW-Authenticate headers.
Windows WebDAV clients receive Basic + Negotiate;
all other clients receive Basic only.

cista/auth.py

@@ -279,7 +279,14 @@ def _log_webdav_user_agent_once(request, user_agent: str):
 
 
 def _build_ua_auth_headers(request, *, include_hint=False) -> dict[str, str]:
-    return {}
+    user_agent = request.headers.get("user-agent", "")
+    _log_webdav_user_agent_once(request, user_agent)
+    if _is_windows_auth_client(user_agent):
+        challenge = f'Basic realm="{_AUTH_REALM}", Negotiate'
+    else:
+        challenge = f'Basic realm="{_AUTH_REALM}"'
+    headers = {"WWW-Authenticate": challenge}
+    return headers
 
 
 def _cleanup_ntlm_challenges():

tests/test_files_auth.py

@@ -159,11 +159,11 @@ async def test_options_unauthenticated_allowed(client):
 
 
 @pytest.mark.asyncio
-async def test_unauthenticated_sends_no_auth_challenge(client):
+async def test_unauthenticated_sends_basic_auth_challenge(client):
     _, res = await client.request("PROPFIND", "/files/")
 
     assert res.status_code == 401
-    assert "www-authenticate" not in res.headers
+    assert res.headers.get("www-authenticate", "").lower().startswith('basic realm="cista"')
 
 
 @pytest.mark.asyncio