Release the main window's native window when it is closed

[ThomasWaldmann]

Stacked on #2551: the first commit here is that PR's. Only the second commit ("Release the main window's native window when it is closed") belongs to this PR; once #2551 is merged this one reduces to it.

Description

Closing the main window only hides it. Qt keeps the native window and its backing store alive for as long as the widget exists — i.e. for the rest of the session once the user has opened the window and put Vorta back into the tray. On a Retina display that is 19–28 MB (depending on the window size) of the ~110 MB Vorta idles at after #2551.

MainWindow.closeEvent now schedules QWidget.destroy() for right after the close has gone through (Qt hides the widget only after closeEvent returns, hence the zero-timer). show() creates the native window again; the window comes back at its previous position and size, and nothing in Vorta touches the native handle (winId/windowHandle are unused).

Tearing the whole MainWindow down instead was measured too: it saves only ~3 MB more (the widgets and models) for a 140 ms rebuild plus all the app.main_window references, so it is not worth it.

Related Issue

Follow-up to #2551 (from the investigation for borgbackup/borg#10328).

Motivation and Context

Vorta is a tray-resident app; users open the window, start or check a backup, close it, and then it sits in the tray for days. The closed window's backing store was the largest remaining single item after #2551.

How Has This Been Tested?

• New test tests/unit/test_main_window.py: the native window is gone after close() and back after reopening via open_main_window_action(). It mocks is_system_tray_available so closeEvent takes the tray path on headless CI. Passed on macOS with PyQt6 6.6.1 and 6.10.0, and on Linux/offscreen (Debian, Python 3.12, in a container).

• Full unit suite on macOS: 266 passed, 7 skipped.

• Footprint measured on macOS 15.7 with PyQt6 6.10 (the shipped stack), real event loop, window closed into the tray:

  	physical footprint	IOSurface
  window shown	108 MB	19 MB
  closed, before	108 MB	19 MB
  closed, with this change	89 MB	0

• Position/size check: window moved and resized, closed, reopened from the tray → same position and size.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.

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