The matrix-account-login endpoint returns failures as a bare plain-text
body today (hive-c0re error_response). The RFC 9457 rework moves it to
application/problem+json. The submit handler previously called
resp.json() on the error path, which threw on the plain-text body and
collapsed every real failure to a generic 'login failed (HTTP 500)',
hiding the actual reason.
Read the error body shape-agnostically: parse JSON only on 2xx for the
success envelope; on failure read the body once as text and, if it
parses as JSON, surface problem+json 'detail' (then 'error'/'title'
fallbacks), else use the raw text. This handles both the current
plain-text and the future problem+json shapes with no BE/FE merge-order
coupling. Header contract doc updated to match.
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