Audit frontend error messages for clarity and accuracy

# Context

Distinct failure modes collapse into single generic toasts. Extraction and compliance evaluation both show "Extraction failed. Please try again." / "Compliance evaluation failed." for any non-2xx response — including the 30s client-side timeout (`SERVER_REQUEST_TIMEOUT_MS` in `frontend/src/server/api-client.ts`), where the backend is still running and "try again" only triggers a second timeout.

The pattern is broader than these two surfaces: wording does not distinguish causes, and recovery advice is given without checking whether it applies.

Related: #679 — async jobs eliminate the timeout case for extraction/compliance; this issue is the wider review.

# Outcome

Error messages distinguish causes where the user's response should differ, and do not suggest actions that are unlikely to help.

# Acceptance criteria

- Current user-facing error messages are inventoried
- Client-side timeouts are distinguishable from backend errors
- Recovery advice appears only where it is actionable
- Wording identifies what failed in terms a user can act on
- Tone and structure remain consistent across surfaces