Fix chat scrolling during streaming

[Blaizzy]

Summary

• Stop automatic transcript following as soon as the user begins scrolling.
• Preserve the user's position through tracking and deceleration.
• Resume following only when the scroll gesture ends at the transcript bottom.

Root cause

Auto-follow was controlled by a 160-point near-bottom geometry threshold. During streaming, frequent content updates could issue another scroll-to-bottom before an upward gesture moved beyond that threshold, pulling the transcript back down.

Impact

Users can now scroll up and review earlier messages while a response continues streaming. The transcript still follows new text when the user remains at the bottom.

Validation

• make xcode-build
• Full Xcode unit-test scheme: 33 tests passed, 0 failures