Follow-up of https://github.com/zed-industries/zed/pull/59994 deals with

<img width="173" height="39" alt="restart to update"
src="https://github.com/user-attachments/assets/5209e15b-cbf9-4530-b0da-7326c8131448"
/>

button overly appearing.

The PR mentioned fixed the redownload issue, but the title bar code
re-surfaced the button on every update check as `is_updated` in this
semantics means a new Nightly update is ready to be installed:
https://github.com/zed-industries/zed/blob/159246f0083787124160620118ac35df5f0f3754/crates/auto_update/src/auto_update.rs#L171-L173
and the old code used this as "can show the button again" reason after
each recheck.

Instead, track dismissed update state better both in the title bar and
the global auto updater, so no new version checks can trigger this and
no new Zed windows get this button again (as it is now).

Release Notes:

- Improved "Restart to Update" button dismissals