fix(web): reject non-JSON API responses instead of resolving them

[BK1031]

Follow-up to the empty service-account token dialog. Root cause was a missing kerbecs route in prod (Gaucho-Racing/infrastructure#164) — /api/service-accounts/* fell through to the SPA catch-all, nginx answered 200 + index.html, and axios resolved it as success. res.data.token was undefined, so the reveal modal opened with an empty token box and no error anywhere.

• Reject 2xx responses whose content-type isn't JSON, with an ERR_BAD_RESPONSE naming the URL and the content-type it got
• Empty bodies (204, no content-type) still pass through untouched
• Turns any future missing gateway route into a visible error toast instead of a silently blank field

All 30 @/lib/api consumers expect JSON and none override responseType, so nothing legitimately returns non-JSON through this client.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reject responses whose content type is missing

When a 2xx response has a nonempty body but omits Content-Type—for example, due to another misconfigured gateway or proxy—the typeof contentType === "string" condition skips this guard entirely, so HTML or another string is still resolved as a successful API response and recreates the silent failure this change is intended to prevent. Treat a missing header as invalid whenever hasBody is true; the existing body check already allows genuinely empty responses.

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