Check Content-Type before parsing error responses as JSON

[iHiD]

Closes #8633
Closes #8632
Closes #8628

Summary

• When a non-JSON response (rate limit "Retry later", HTML error page, binary data) was thrown by fetchJSON, useErrorHandler in ErrorBoundary.tsx unconditionally called .json() on it, producing a SyntaxError that was reported to Sentry
• Now checks the Content-Type header first and only attempts JSON parsing when the response claims to be JSON — mirrors the same check already used in fetch-json.ts
• Non-JSON error responses skip straight to showing the default error message (same user-visible behavior as before)

Test plan

• yarn test — all 160 suites pass (1551 tests), no regressions
• TypeScript type check — no new errors (same 14 pre-existing errors in react-i18next)
• Pre-commit hooks pass (prettier)

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