fix(ui): api client never parses application/problem+json error bodies

[w7-mgfcode]

Problem

frontend/src/lib/api.ts only treats application/json as a JSON response:

const isJson = contentType.includes('application/json')

All backend errors are RFC 7807 application/problem+json (per app/core/problem_details.py, the project standard since Phase 7). "application/problem+json".includes("application/json") is false, so error bodies are never parsed:

• ApiError.detail is always {} for error responses.
• getErrorMessage() falls back to error.message, which is the raw JSON string of the problem body.
• Every ErrorDisplay in the SPA renders the raw RFC 7807 JSON blob instead of the human-readable detail.

Impact

Repo-wide — affects every error surface in the frontend, not a single page. Latent since Phase 7/10 (#43, #78) because error states are off the happy path and rarely dogfooded.

Discovery

Surfaced while dogfooding the Forecast Explainability slice (#224): the ExplanationPanel on /visualize/forecast renders explanation errors as an expected in-card state, making the raw-JSON leak visible.

Fix

One line — match the +json structured-syntax suffix as well:

const isJson =
  contentType.includes('application/json') || contentType.includes('+json')

Note

The fix ships in the #224 explainability PR (the slice's error UX depends on it). This issue tracks the root-cause repo-wide bug for the record.

[w7-mgfcode]

Fixed in PR #231 — api.ts now matches the +json structured-syntax suffix so application/problem+json error bodies parse into ApiError.detail. Regression test added (api.test.ts).