Custom OpenAI-compatible providers surfaced Node's opaque
`TypeError: fetch failed` whenever the upstream HTTP call failed
(#121) — DNS, TLS handshake, connection refused, timeout — all
collapsed to the same useless string. The actual reason was already
on `error.cause` as a SystemError but never reached the user.
Add `formatFetchError()` that walks the cause chain (including
AggregateError emitted when undici tries multiple A records and each
attempt fails) and prefixes the SystemError code so users see
`ENOTFOUND: getaddrinfo ENOTFOUND api.foo.com` or
`ECONNREFUSED: connect ECONNREFUSED 127.0.0.1:443` instead of
`fetch failed`. Wire it into the model-list proxy (most common
trigger from the AI Settings dialog) and the builtin chat stream.
Closes#121
4ed1203bf1