Fix streaming reasoning UI jank / 修复流式思考过程导致的界面卡顿

[SivanCola]

Summary

• Restore the compact-mode streaming contract so reasoning does not auto-expand while compact steps are folding intermediate work.
• Mount reasoning content only while the reasoning section is expanded.
• Coalesce transcript auto-scroll work into one animation frame during fast streaming updates.
• Add an opt-in streaming reasoning tail preview for compact mode while keeping standard mode's full live reasoning display.

Verification

• Ran reasoning display helper assertions with Node.
• Ran git diff --check.
• Ran frontend CSS syntax and z-index token checks.
• Parsed desktop/frontend/package.json successfully.

Full frontend tsx/tsc checks were not run because frontend dependencies are not installed in this worktree.

[SivanCola]

@codex review

[chatgpt-codex-connector]

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[SivanCola]

approve