feat(web): steer queued messages into the running turn from the queue UI

What feature would you like to see?

Problem
In the web UI, messages sent while a turn is running are queued and only delivered after the turn ends. The TUI lets users inject input into the running turn with Ctrl+S (steer), and the web composer supports the same shortcut — but there is no visible, discoverable affordance for it, and no way to steer a single queued message without steering everything (composer draft + whole queue) at once.

Proposal
Add explicit “Steer now” actions to the queued-message list:

A per-message “Steer now” button on each queued row — injects only that message into the running turn, leaving the rest of the queue untouched.

A queue-header “Steer now” button — injects the whole queue (same merge semantics as Ctrl+S).

Behavior details:
Falls back to a normal send when no turn is running.
On a definitive server rejection the message is restored at its original queue position; on ambiguous failures it is not re-queued, to avoid duplicate sends.
Attachments on queued messages are carried along.

Notes
No backend changes needed — POST /sessions/{id}/prompts:steer already exists.

Additional information

No response

Locate the web queued-message list and composer code, then trace the existing POST /sessions/{id}/prompts:steer path and queue send handling. Implement per-message and queue-header actions while preserving attachments and the specified rejection behavior. Done means normal send is used when no turn runs, single-message steering leaves the rest queued, and whole-queue steering matches Ctrl+S semantics.