Allow queueing messages to a working thread on mobile

[radroid]

What to build

On mobile there is currently no way to queue a message to a thread while it is actively working. On desktop this is possible, but on mobile the equivalent affordance is missing:

• The compose area shows a step button (not a send/queue button) while the thread is working.
• Pressing Enter on the mobile keyboard just inserts a newline, so there's no keyboard path to queue either.

As a result, mobile users cannot line up a follow-up message to be picked up once the current turn finishes — they have to wait for the thread to stop before they can send.

We want mobile to support queueing a message to a working thread end-to-end, matching the desktop behavior.

Acceptance criteria

• While a thread is working on mobile, the user can queue a message that is delivered/processed once the current turn completes.
• There is a clear, discoverable control to queue the message (e.g. a send/queue button available alongside or instead of the step button), sized appropriately for touch.
• Queued messages are visibly indicated in the UI so the user knows the message is pending.
• Behavior is consistent with how queueing works on desktop.

Blocked by

None - can start immediately