Add raw API response viewer per message

A \`{ }\` button on each assistant message opens a modal showing the full raw SSE chunks from that turn. Useful for debugging, analysis, and inspecting logprobs/usage.

Reference: [tool-calling-demo](https://github.com/rdwj/tool-calling-demo/blob/main/calculus-ui/static/app.js)
See [UPSTREAM_PORT.md](https://github.com/rdwj/tool-calling-demo/blob/main/UPSTREAM_PORT.md) section 1.4.