feat(input): Alt/Option+Enter sends follow-up while streaming

[dingxiaobo]

Summary

Add a keyboard shortcut to send a follow-up message while the agent is still running.

Previously, the only way to queue a follow-up was clicking the purple Follow-up button. Enter during streaming always defaulted to steer (falling back to follow-up only when steer was unavailable), so there was no one-key route to follow-up.

Now Alt/Option+Enter sends the composed message as a follow-up (queued until the agent finishes), matching the existing Enter→steer default.

Changes

• components/ChatInput.tsx — new keydown branch: Enter + altKey (while streaming, not IME-composing, onFollowUp available) dispatches sendQueued("followup").
  • e.altKey covers Option on macOS and Alt on Windows/Linux with one path.

Behavior

State	Key	Action
streaming	Enter	steer (default; follow-up if steer unavailable)
streaming	Alt/Option+Enter	follow-up
idle	Alt/Option+Enter	normal send (same as Enter)