Queue chat sends while agent is busy

[Mingwwww]

Summary

• allow composing messages while an agent turn is running
• queue a busy send for automatic delivery when the current turn completes
• allow clicking send again to force-send by interrupting the current turn
• isolate streaming/thinking render state by run id to avoid stale turn bleed-through
• add a cancel action for queued busy sends

Testing

• git diff --check
• manual verification in Vite dev UI at http://localhost:5173/p/general