test: dispose Qt test objects before shutdown

[lukisch]

Summary

• Use one session-scoped QApplication for GUI tests and dispose test-owned widgets and tray icons before interpreter shutdown.
• Parent only test-owned tray objects to the shared application; preserve functional assertions and the standalone asset-test entry point.
• This is a controlled test-lifecycle candidate for the intermittent Windows native exits tracked in Intermittent Windows pytest process abort after passing suite #9. It does not establish or claim to fix their native cause.

Validation

• Python 3.12.10, pytest 9.1.1, PySide6/shiboken6 6.12.0: 104 passed and 6 subtests passed; real process exit 0 in an isolated profile.
• Python 3.13.15 without PySide6: source-platform smoke contract, 1 passed; real process exit 0.
• Standalone asset test: 6 tests passed; real process exit 0.
• Ruff and git diff --check passed.

Refs #9