Fix auto update UI behavior

[BitRyuu]

Summary

• show a notification when a manual update check finds the app is already up to date
• show progress, restart-ready, and error notifications for manual update checks

Testing

• cargo check -p auto_update_ui
• ./script/clippy

Release Notes:

• Improved manual auto update notifications in the UI

[cla-bot]

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[BitRyuu]

[BitRyuu]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[SomeoneToIgnore]

Thank you, we're not going with this.

1. This is a visual change, but has no screenshots and videos (the latter seems more appropriate in this case) despite https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#sending-changes
2. There is no prior discussion or issue — what's "fixed", why the release notes have "improved" instead???
3. Notifications are flashy, bad and distracting usually, with us trying to use them sparingly — I suspect this won't look good in this case either