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/diff_render.rs

@@ -146,7 +146,7 @@ fn render_changes_block(rows: Vec<Row>, wrap_cols: usize, cwd: &Path) -> Vec<RtL
     let total_removed: usize = rows.iter().map(|r| r.removed).sum();
     let file_count = rows.len();
     let noun = if file_count == 1 { "file" } else { "files" };
-    let mut header_spans: Vec<RtSpan<'static>> = vec!["• ".into()];
+    let mut header_spans: Vec<RtSpan<'static>> = vec!["• ".dim()];
     if let [row] = &rows[..] {
         let verb = match &row.change {
             FileChange::Add { .. } => "Added",