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/mod.rs

@@ -716,8 +716,8 @@ mod tests {
             top.push(buf[(x, 1)].symbol().chars().next().unwrap_or(' '));
         }
         assert!(
-            top.trim_start().starts_with("Working"),
-            "expected top row to start with 'Working': {top:?}"
+            top.trim_start().starts_with("• Working"),
+            "expected top row to start with '• Working': {top:?}"
         );
         assert!(
             top.contains("Working"),