feat(chat): add queued composer steering

[fschrhunt]

Summary

• add local queued composer messages while Huma is active
• render a Codex-style queue deck above the composer with Steer, delete, edit, follow-up, and move controls
• steer selected queued cards through chat.send with stable idempotency keys
• auto-send follow-up-only cards after the active run ends

Verification

• swift test
• ./script/build_and_run.sh --verify

Risk

• Moderate. This changes composer behavior during active runs and adds local queue state, but normal idle sends remain covered by tests.