The AI chat error banner always showed a generic "Something went wrong"
with no reason. The server already forwards the provider cause into the
stream (e.g. "Cannot connect to API: read ECONNRESET"), but the client
hid it behind a static heading.
- describeChatError now returns { title, detail }: a short heading naming
the cause category plus a one-line explanation.
- Add classifyProviderError: maps connection reset, timeout, rate limit,
context-window overflow, quota and auth failures to clear categories;
the 403/503 gating responses are preserved; unknown errors fall back to
the verbatim provider text.
- Match HTTP status codes only as the leading token and textual signatures
only against the message head (before "| response body:"), so a number
or phrase in the response-body snippet never mislabels the cause.
- Use the new {title, detail} in all three banners: chat-thread,
share-ai-widget and the persisted-error banner in message-item.
- Cover the classifier with 20 unit tests (categories + regressions).
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