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():