Introduce a new "check_app_updates" configuration (default: true) to allow users to enable/disable automatic YTSage update checks. Adds a styled checkbox to the Updater tab with getter, loads the setting (defaults to enabled for older configs), and saves it from the custom options dialog. The main update check now respects this setting and will skip checking when disabled. Also include English localization entries for the new UI strings.
Introduce an opt-in beta updates feature: add a styled "Receive Beta Updates" checkbox in the Updater UI with i18n strings, load/save logic in the custom options dialog, and a default config key (check_beta_updates=false).
Add _fetch_github_beta_version to the update thread to query GitHub releases (selecting the highest tag) and emit update_available for newer beta builds. When beta checking is enabled, the thread will perform the GitHub beta check and return early to avoid conflicting with the PyPI check. This enables users to opt into preview releases safely.
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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