[Bug][GUI] Stop button disappears (replaced by Send) after switching conversations while a turn is running — running turn becomes unstoppable

Description

While a turn is running, the composer shows a Stop button so the user can interrupt the agent. If the user switches to another conversation and then switches back to the running one, the Stop button is gone — the composer shows the normal Send button instead, as if the session were idle. The backend turn is still executing (sending a new message fails with "This session is already running a turn. Wait for it to finish or stop it."), so there is no UI control left to stop the running turn.

Steps to reproduce

1. Open a conversation and start a task that takes a while (a normal long agent turn).
2. While it is still running, switch to a different conversation.
3. Switch back to the first conversation.
4. The composer no longer shows the Stop button — it shows the Send button.

Additional symptom confirming the turn is still alive:

• Sending "stop" as a message in that conversation fails with: This session is already running a turn. Wait for it to finish or stop it.

Actual behavior

Switching away and back does not restore the Stop control, even though the turn is still executing. The user is left with no way to interrupt the turn from the UI.

Expected behavior

Returning to a conversation whose turn is still running should restore its live running state, including the Stop button, exactly as it appeared before switching away. Switching conversations should never remove the ability to interrupt a running turn.

Environment

• Windows 11
• OpenWorker desktop app (local-first build), GUI surface
• Exact app version not captured at report time

Related: #311 "Running session loses visible thinking state when switching between two active sessions" — same root area (session-switch state not restored in the GUI); this issue is specifically about the lost Stop control / inability to interrupt a still-running turn.