Improve conversation input and activity presentation

[longquanzheng]

Summary

• allow request-input headers up to 64 Unicode characters while retaining runtime validation and documenting the agent-facing limit
• distinguish user and assistant messages, reveal timestamps without layout shift, and reduce supporting Work log typography
• surface failed input requests and place compaction activity in its corresponding conversation turn, including timestamp-based fallback for older history

Verification

• make check
• make test-full-stack-e2e (20 tests against a real Dex server)
• targeted real-Dex compaction integration test