Add option to 'Check for Updates' on demand, not just automatically

[Anutrix]

Problem Description

I turn off 'Check for updates automatically', because I don't want to update Rancher Desktop without explicit action.

But there's no way to check for updates manually when I do need it.

Proposed Solution

Add a button that says 'Check for updates' anywhere in the application.

The code logic for it already exists for the automatic case.

Additional Information

No response

[mook-as]

Note for implementation:
We actually apply the update on next start (since we trigger a restart on downloading the update, and then actually run it later). We may need to check that we can actually apply the update if it's staged when updates are disabled.