tui: • Working, 100% context dim (#4629)

- add a `•` before the "Working" shimmer
- make the percentage in "X% context left" dim instead of bold

<img width="751" height="480" alt="Screenshot 2025-10-02 at 2 29 57 PM"
src="https://github.com/user-attachments/assets/cf3e771f-ddb3-48f4-babe-1eaf1f0c2959"
/>

codex-rs/tui/src/bottom_pane/footer.rs

@@ -222,7 +222,7 @@ fn context_window_line(percent: Option<u8>) -> Line<'static> {
     let mut spans: Vec<Span<'static>> = Vec::new();
     match percent {
         Some(percent) => {
-            spans.push(format!("{percent}%").bold());
+            spans.push(format!("{percent}%").dim());
             spans.push(" context left".dim());
         }
         None => {