fix: fix border style for BottomPane (#893)

This PR fixes things so that:

* when the `BottomPane` is in the `StatusIndicator` state, the border
should be dim
* when the `BottomPane` does not have input focus, the border should be
dim

To make it easier to enforce this invariant, this PR introduces
`BottomPane::set_state()` that will:

* update `self.state`
* call `update_border_for_input_focus()`
* request a repaint

This should make it easier to enforce other updates for state changes
going forward.

codex-rs/tui/src/status_indicator_widget.rs

@@ -120,7 +120,7 @@ impl WidgetRef for StatusIndicatorWidget {
             .padding(Padding::new(1, 0, 0, 0))
             .borders(Borders::ALL)
             .border_type(BorderType::Rounded)
-            .border_style(widget_style);
+            .border_style(widget_style.dim());
         // Animated 3‑dot pattern inside brackets. The *active* dot is bold
         // white, the others are dim.
         const DOT_COUNT: usize = 3;