Fix: prevent crash when closing multiple profiles at once

Changes -

• Audited all uses of QtConcurrent::run and QFutureWatcher throughout the codebase.
• Refactored all async lambdas and slots that could access deleted QObjects:
  • Used QPointer to safely guard access to any QObject (Host, dialogs, etc.) in lambdas, ensuring the lambda checks if the object is still alive before accessing it.
  • Parent all QFutureWatcher instances to the relevant QObject, so they are auto-deleted if the parent is destroyed.
  • In critical cases, ensured destructors wait for async operations to finish before allowing the object to be destroyed.
• Applied this robust pattern to all at-risk locations:
  • Profile save operations (Host.cpp)
  • XML export operations (XMLexport.cpp)
  • Package export operations (dlgPackageExporter.cpp)
  • Theme download and unzip operations (dlgProfilePreferences.cpp)
• Reviewed and updated all related code to ensure no use-after-free or double-free is possible, even under rapid profile closing or shutdown.
• Confirmed by user feedback and code audit that the crash is no longer reproducible.

How the Bug Was Fixed

• Root cause: Async operations (like saving profiles or exporting packages) could finish after their parent objects were destroyed, leading to use-after-free crashes.
• Solution:
  • All async callbacks now use QPointer to check object validity before accessing members.
  • All QFutureWatcher objects are parented to the relevant QObject, ensuring proper cleanup.
  • Destructors wait for async operations to finish if necessary.
• Result: No more crashes when closing multiple profiles at once, even under heavy or rapid usage. Solves - #7478 /claim #7478