Added a response-time badge to the playground's Technical view

Every assistant message in the playground's Technical view now shows a small badge with the turn's response time (e.g. 164ms), next to the "more" menu. Hover or focus it to see the breakdown: time to first token, total time, an estimated speed in tokens/sec, and the number of streamed chunks.

There's no cost figure. The speed shown is an estimate based on response length, not the exact token count from the model provider.

The badge only appears in the Technical view. It's absent from the User view (what a shopper sees) and from the public agent link, and it doesn't show up when you reopen a past conversation under Conversations: timing is measured live while a response streams in, so there's nothing to show for a conversation you're just re-reading.