Add settings controls for update checks

[beNative]

Summary

• add controls in the General settings section to toggle automatic update checks, trigger manual checks, and clarify pre-release downloads
• extend settings persistence and renderer effects with the new auto-update preference
• update the Electron preload and main processes to respect the stored preference, expose manual checks, and read settings from the database

Testing

• npm run build (fails: tailwindcss CLI is unavailable in the container)

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https://chatgpt.com/codex/tasks/task_e_68f374fa9ac083329bc75f27aa1a030c