fix(api): remove duplicate /api/traces endpoint that broke React UI (#9427)

The API Traces tab in /app/traces always showed (0) traces despite requests
being recorded.

The /api/traces endpoint was registered in both localai.go and ui_api.go.
The ui_api.go version wrapped the response as {"traces": [...]} instead of
the flat []APIExchange array that both the React UI (Traces.jsx) and the
legacy Alpine.js UI (traces.html) expect. Because Echo matched the ui_api.go
handler, Array.isArray(apiData) always returned false, making the API Traces
tab permanently empty.

Remove the duplicate endpoints from ui_api.go so only the correct flat-array
version in localai.go is served.

Also use mime.ParseMediaType for the Content-Type check in the trace
middleware so requests with parameters (e.g. application/json; charset=utf-8)
are still traced.

Signed-off-by: Pawel Brzozowski <paul@ontux.net>
Co-authored-by: Pawel Brzozowski <paul@ontux.net>