fix(updater): show download progress toast and toast feedback for settings check

[AmethystLiang]

Summary

• Show a real-time download progress toast in the bottom-right corner during update downloads (e.g. "Downloading v1.0.51… 45%")
• Make the Settings "Check for Updates" button use the user-initiated flow so it shows toast notifications (checking, result, errors) — same as the menu "Check for Updates"
• Properly dismiss the download toast on completion or error

Test plan

• Build succeeds
• Validated via Electron CDP: "Check for Updates" in Settings shows toast feedback
• When a real update is available, clicking "Install" shows a progress toast that updates in-place
• Toast dismisses when download completes, replaced by "ready to install" toast

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