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

[jihanurrahman33]

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.

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[jihanurrahman33]

@googlebot I signed it