fix(app): stream the reply draft inside the timeline instead of over it

[iamnbutler]

The streaming reply was rendered as an absolutely positioned, unstyled overlay pinned to the bottom of the chat. Long drafts covered the transcript as raw markdown. The draft now joins the timeline as a streaming agent message, labeled with the chat's model, so it is laid out, formatted, and scrolled like other messages, and the final reply replaces it.

Smoke-tested against a separate ace serve and Vite with a long Opus reply: mid-stream, the draft rendered inside the agent message, pinned to the bottom with no overlap, and the final message replaced it cleanly.

Built in Ace

The code, checks, and commit were done in Ace. For the visual smoke test:

• Playwright with Chrome for Testing opened the app and took screenshots. Accessibility inspection of the Chrome window failed, so Ace could not click into the page (#113, and browser page inspection is Inspect and debug browser pages from an Ace channel #50).
• Preview displayed those screenshots so they could be captured with pixel inspection, because the agent can't read image files by path (Let native agents inspect image files by path #51).