fix: shut down worker QThreads safely on close

[JeanExtreme002]

Summary

Closing a dialog or the main window while a background worker QThread was still running could abort the whole process with QThread: Destroyed while thread is still running, or deadlock on the BlockingQueuedConnection the scan workers use to hand chunks back to the GUI thread.

This PR makes worker-thread teardown safe across the app.

Changes

• Add a shutdown_worker_thread() helper that cancels the worker, disconnects every signal up front, and detaches any worker still wedged in a backend call when the wait expires — held in a module-level list so the closing dialog never destroys a live thread.
• Pump the event loop while waiting on the scan worker, so the BlockingQueuedConnection chunk_ready emit can be serviced instead of deadlocking.
• On main window close, stop the heartbeat first (before any event-loop pumping) and close worker-bearing auxiliary dialogs (memory map, threads, modules, pointer scan) so each joins its own thread.
• Keep the old process handle open until the old cheat poller has been joined when switching targets, avoiding reads from a closed handle.
• Guard cellChanged against items already cleared mid-teardown.