Two improvements:
1. The progress ToolTrail and the streaming MessageLine were two
sibling JSX blocks in appLayout with hand-rolled margin glue
between them. Extracted into `<StreamingAssistant>`, a single
component that owns both the trail and the streaming body plus
the 1-row gap between them. appLayout just hands it `progress`
and theme; the layout logic lives in one place, matching the
mental model that these two pieces are one live assistant turn.
2. Thinking token label was hidden when `reasoningTokens === 0` even
if the live reasoning text was already populated (the
scheduleReasoning timer hadn't ticked, or the model sent no
reasoning but the text was coming in via reasoning.delta).
Changed the tokenCount fallback from `reasoningTokens !==
undefined ? reasoningTokens : estimate` to `reasoningTokens > 0 ?
... : estimate` so the label appears the moment text exists.
319aabbb80