Desktop: agent activity row under the composer cycles through old actions instead of showing what the agent is doing now

**Motivation**

When an agent is working, the row under the composer shows what it is doing. Today that row cycles through several recent activity lines on a timer, so what you read is not what the agent is doing right now — an action from earlier in the turn can be on screen while something else is running.

That makes the row hard to follow. You cannot tell whether the agent just started, is deep into a long step, or is nearly done, and the same few lines repeating gives no sense of progress.

**Proposed solution**

Show the current activity in real time instead of cycling: one line that updates in place as the agent moves from step to step, always reflecting the latest state.

**Alternatives considered**

Slowing the cycle down does not help — the problem is that older entries are shown at all, not how fast they change. The full history is already available in the agent session view for anyone who wants it.

**Additional context**

Closest existing work: PR #4662 ("stable Slack-style status line in the composer activity bar") proposes replacing the rotation with a single line that updates in place, which is the same request. It is open and has had no review since it was opened. No issue found for this.

Review open PR #4662 first, since it covers the same composer activity-bar request and is the closest existing work. Confirm whether it implements a single in-place-updating status line rather than rotating older activity entries; the issue is done when the row always reflects the latest agent activity.

Written by the indexing model from the issue text.