RTL: Flip directional icons, like `chevron_right` and `send`, in RTL mode

For example, we should flip the "send" paper-airplane button in the compose box when it's rendered in RTL mode:

<img width="445" height="120" alt="Image" src="https://github.com/user-attachments/assets/ba157839-8638-44fc-8afd-07599a9629a4" />

## Implementation

- [`IconData.matchTextDirection`](https://api.flutter.dev/flutter/widgets/IconData/matchTextDirection.html) might help with this.