Fixes appcast using wrong platform

[ahmet-fennel]

Important

Fixes platform mismatch in appcast by adding platform check in parseItemsFromXMLString() in appcast.dart.

• Behavior:
  • Added _isCorrectPlatform() function in appcast.dart to check if the platform matches the current operating system.
  • Modified parseItemsFromXMLString() to use _isCorrectPlatform() for filtering items based on platform.
• Formatting:
  • Minor formatting changes for better readability in appcast.dart.

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