Separate turn prompt from nested activity

[VolkerChristian]

Summary

• add a visibly distinct canonical 8 px structural section gap between complete You-prompt content and the first nested turn card
• reserve no gap on the initial prompt-only card; reveal it only when the first nested card arrives, retaining the same You-card widget
• align CodexWUI nested-card spacing and keep whitespace out of authored Markdown
• document and regression-test both initial and first-nested-card geometry

Validation

• QT_QPA_PLATFORM=offscreen codexui-conversation-cards-test
• native codexui-conversation-cards-test and codex-ui build
• visual capture of the real Qt card fixture confirms the painted separation
• npm test (10/10 WebUI suites)