fix: show the actual error instead of a generic message when a request fails

[androemeda]

Summary

Chat and biomodel analysis reported every failure with the same generic line, hiding the reason the backend actually gave. Both now show it.

The bugs

Chat — use-chat-history.tsx never checked res.ok. A rejected request still parses as JSON, so data.response came back undefined and fell through to "Sorry, I didn't get a response from the server." — while the real reason sat unread in data.detail. This affected every failure: rate limits, bad requests, and LiteLLM budget exhaustion, which is the one users are most likely to hit. It read like a server glitch, so the natural response was to retry, which failed the same way.

Analyze — analyze/[id]/page.tsx threw away detail on a failed response, and its catch then overwrote the message with a hardcoded "Failed to analyze biomodel.", so both halves needed fixing.

Changes

• New lib/api-error.ts — reads FastAPI's detail (string from HTTPException, or a list of objects from request validation), falling back when the body is missing or isn't JSON.
• use-chat-history.tsx — check res.ok; show the reason, or The server returned an error (<status>). when there's no usable body. Network failures and cancellation keep their existing messages, which were already accurate.
• analyze/[id]/page.tsx — throw the extracted message and preserve it through the catch.
• profile/page.tsx — dropped a byte-identical local copy of the helper in favour of the shared one. No behaviour change.

VCMLSection and DiagramSection already handled this correctly and are untouched.