Introduce SmoothTabWidget and a FadingStackedWidget to provide cross-fade transitions between tabs. Added ytsage/gui/ytsage_smooth_tab_widget.py which implements a QTabBar + FadingStackedWidget (default 300ms fade) and overlay-based fade logic. Updated ytsage/gui/ytsage_gui_dialogs/ytsage_dialogs_custom.py to use SmoothTabWidget instead of QTabWidget and adjusted the stylesheet selector to target the new content frame (#tabContent) so existing styles continue to apply.
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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