Studio: render generated images inline for the Images pill (#5705)

The pill wired the request end of the loop but the response was lost
on the client: the backend emits a `tool_end` _toolEvent carrying the
base64 PNG on `image_b64` / `image_mime`, but the chat-adapter only
read the `result` string and the generic ToolFallback printed the
prompt as JSON args with an empty Result block -- the "I see no
image" symptom in the chat.

- chat-adapter: when the closing `tool_end` is for `image_generation`,
  repackage `image_b64` + `image_mime` (+ size/quality/background)
  into a structured result object instead of dropping them.
- New `ImageGenerationToolUI` reads that result and renders the image
  inline via `<img src="data:image/...;base64,...">` with the prompt
  as a caption. Falls back to a spinner while the request is still
  running.
- Register the component under `image_generation` in thread.tsx's
  tools.by_name map so it preempts ToolFallback for this tool only.