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.
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