fix(frontend): incorrect json detection for `content-type` with charset causes wrong response parsing

[huyhoang171106]

Description

getResponseBody() checks Content-Type using strict equality to 'application/json'. Real APIs commonly return 'application/json; charset=utf-8'. In that case, the function falls back to response.text() and returns a string instead of parsed JSON, causing downstream runtime failures when callers access object properties.

Severity: high
File: src/frontend/js/api/utils.ts

Expected Behavior

The code should handle this case properly to avoid unexpected errors or degraded quality.