uploadPending() read `data.files` from /api/upload without checking `res.ok`, so
a non-OK response (429 rate limit, 413 too large, …) was swallowed: the pending
files vanished and the chat sent with no attachments and no feedback — part of
why the model "didn't even see them" in #1346.
Check res.ok; on failure show the server's reason via a toast and keep the
pending files so the attach strip re-renders for a retry (matching the existing
"restored on error" comment that the code never actually honored).
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
