docs(readme): replace pub.dartlang.org with pub.dev; normalize macOS; update Flutter docs link (#728)

Small documentation cleanup in README:
- Replace legacy `pub.dartlang.org` links with `pub.dev`
- Normalize platform spelling `MacOS` -> `macOS`
- Update `flutter.dev/docs/...` links to `docs.flutter.dev/...`

No code changes.

Co-authored-by: jihanurrahman33 <[email protected]>

Author: jihanurrahman33

README.md

@@ -1,6 +1,6 @@
 # Fluwx
 
-[![pub package](https://img.shields.io/pub/v/fluwx.svg)](https://pub.dartlang.org/packages/fluwx)
+[![pub package](https://img.shields.io/pub/v/fluwx.svg)](https://pub.dev/packages/fluwx)
 ![Build status](https://github.com/OpenFlutter/fluwx/actions/workflows/build_test.yml/badge.svg)
 [![GitHub stars](https://img.shields.io/github/stars/OpenFlutter/fluwx)](https://github.com/OpenFlutter/fluwx/stargazers)
 [![GitHub forks](https://img.shields.io/github/forks/OpenFlutter/fluwx)](https://github.com/OpenFlutter/fluwx/network)