[Tour of Beam] [Frontend] [Task]: Return flutter_markdown package after text selection is fixed

### What needs to happen?

The [flutter_markdown](https://pub.dev/packages/flutter_markdown) package was temporarily replaced with [flutter_markdown_selectionarea](https://pub.dev/packages/flutter_markdown_selectionarea), because text selection [doesn't work properly in flutter_markdown](https://github.com/flutter/flutter/issues/99819).

Replace flutter_markdown_selectionarea with flutter_markdown when the issue is resolved.

### Issue Priority

Priority: 3 (nice-to-have improvement)

### Issue Components

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