Fix startup UI visibility during deferred model download

[thomaswantstobeaskeleton]

Motivation

• On some systems opening a modal model-download dialog directly in MainWindow.showEvent can block the window's first paint, leaving logs and downloads complete but no visible UI, so the initial deferred download should be queued after the event loop gets a chance to paint.

Description

• Replace the direct call to self._run_deferred_model_download() with QTimer.singleShot(0, self._run_deferred_model_download) and add a clarifying comment in ui/mainwindow.py so the modal progress dialog is launched after the main window is shown.

Testing

• Ran python -m compileall -f -q ui/mainwindow.py and python launch.py --help, both commands succeeded.

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Codex Task