Polish conversation card identity colors

[VolkerChristian]

Summary

• use violet for user messages and canonical blue for final Codex answers
• keep interim Codex updates and supporting activity cards neutral
• keep the animated pending prompt in the violet user-message palette through acknowledgment
• apply the same visual contract to native CodexUI and the web UI
• document the violet palette and message identity rules

Verification

• native CodexUI build
• codexui-conversation-cards-test
• web TypeScript build and all 7 web test suites

The sandbox-only socketpair and shell integration restrictions are unchanged; affected UI and presentation tests pass.