Add a safe blurred-overlay flow for modal dialogs by implementing run_dialog_with_blur and _apply_blur_to_pixmap on YTSageApp. The new flow captures a window screenshot, generates a blurred/dimmed pixmap via a QGraphicsScene + QGraphicsBlurEffect, shows it in an overlay QLabel with a fade-in, runs the dialog, and then removes the overlay.
Replace many direct dialog.exec() calls with run_dialog_with_blur(...) across the main GUI (download settings, about, history, playlist selection, custom options, cookie login, ffmpeg check, time range, setup success dialogs, etc.). Also add imports required for pixmap/graphics handling.
Make animate_widget_fade_in usage safer by checking for the method before calling it and falling back to setVisible(True) for format table and thumbnails to avoid QPainter/graphics-effect conflicts. Small comment/cleanup in fade-in/out methods.
Hide the thumbnail QLabel before assigning the pixmap and trigger animate_widget_fade_in to smoothly fade the thumbnail into view. Keeps existing exception logging for thumbnail processing.
Moved all files from src/gui/ to ytsage/gui/ and updated import statements to use relative imports within the new package structure. This improves modularity and prepares the codebase for distribution as a proper Python package.
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