fix(tui): flush active cell before inserting steer message in transcript

Steered/queued user messages were being inserted into app.history
before the active cell (holding streaming thinking/tool content) was
flushed, causing the user's message to render above (before) the
thinking block that chronologically preceded it.

Now call app.flush_active_cell() before app.add_message() in
steer_user_message(), matching the pattern used in MessageStarted
and MessageDelta handlers.

Fixes #2225.

crates/tui/src/tui/ui.rs

@@ -5384,6 +5384,11 @@ async fn steer_user_message(
     engine_handle.steer(content.clone()).await?;
     app.last_submitted_prompt = Some(message.display.clone());
 
+    // Flush any streaming thinking/tool content into history before
+    // inserting the steer message, so the steer appears after (below)
+    // the content that chronologically preceded it.
+    app.flush_active_cell();
+
     // Mirror steer input in local transcript/session state.
     app.add_message(HistoryCell::User {
         content: format!("+ {}", message.display),