Pointing AddRemoteServer at a Piped Vue SPA (e.g. the frontend host
rather than the API host) used to fail with a generic
"could not detect instance type" — every JSON probe got the same
index.html back. On the failure path, fetch `/` once more and look
for `<title>Piped</title>`; if matched, return a new
`pipedFrontendDetected` error that tells the user to enter the
Piped API URL instead.
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