Use Option symbol for mac key hints (#5582)

## Summary
- show the Option (⌥) symbol in key hints when the TUI is built for
macOS so the shortcut text matches the platform terminology

## Testing
- cargo test -p codex-tui

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https://chatgpt.com/codex/tasks/task_i_68fab7505530832992780a9e13fb707b

codex-rs/tui/src/key_hint.rs

@@ -6,6 +6,9 @@ use ratatui::style::Style;
 use ratatui::style::Stylize;
 use ratatui::text::Span;
 
+#[cfg(target_os = "macos")]
+const ALT_PREFIX: &str = "⌥ + ";
+#[cfg(not(target_os = "macos"))]
 const ALT_PREFIX: &str = "alt + ";
 const CTRL_PREFIX: &str = "ctrl + ";
 const SHIFT_PREFIX: &str = "shift + ";