Collapsible reasoning in TUI

[krissetto]

With this PR reasoning content is rendered by default in a collapsed block to save space in the UI, especially useful when models decide to overthink.

Collapsed reasoning blocks:

• are expandable to their full content by clicking on the block's "header" (where "Thinking" is)
• show only the last 3 lines of content, with the first one faded to indicate there's more to see above
• show tool calls only while in progress. full view can be seen when expanded

Screencasts

before:

Screencast.From.2026-01-18.14-15-47.mp4

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after:

Screencast.From.2026-01-18.14-19-04.mp4