storing credits (#6858)

Expand the rate-limit cache/TUI: store credit snapshots alongside
primary and secondary windows, render “Credits” when the backend reports
they exist (unlimited vs rounded integer balances)

codex-rs/tui/src/chatwidget/tests.rs

@@ -81,6 +81,7 @@ fn snapshot(percent: f64) -> RateLimitSnapshot {
             resets_at: None,
         }),
         secondary: None,
+        credits: None,
     }
 }