Usability improvements: refresh, sign-in and helper recovery - #874

After changing the data source, signing in, or selecting an Xcode in Terminal, Xcodes can show stale state. Helper failures also leave users without a clear next action. This change refreshes the relevant state and provides recovery choices at the point of failure.

Changes

• Refresh available releases once per launch and after sign-in/data-source changes. Preserve a running full refresh when the app becomes active; prevent cancelled requests from fetching or publishing replacement data.
• Recheck the selected developer directory when the app becomes active, independently of catalog refresh, so a Terminal xcode-select change updates the active indicator.
• Explain why the Apple data source requires sign-in and offer Login instead of a raw 401 error. Avoid new-release notifications caused only by switching data sources.
• Retry the version check while a newly installed helper starts, and explain authorization/signature/connection failures.
• For failed helper actions, offer Advanced Settings or a safely quoted command to copy into Terminal. When Xcode is installed but post-install steps fail, offer Open Xcode to finish setup on first launch.
• Label notifications generated by unit tests; include helper/notification warning fixes alongside the affected code. Preserve the upstream signing configuration.

Validation

• Full macOS unit suite: 62 tests passed with Xcode 27.1 beta, Test configuration, arm64 destination, signing disabled.
• Tests cover active-selection refresh during catalog refresh, helper retry/failure, post-install recovery, and shell quoting.
• Fixed the cancellation race in refresh-test cleanup; the complete suite passes without excluding the previously stalled test.
• Combined with the other two topics: all 75 tests passed.

Reviewer checks

1. Switch data sources and sign in with the Apple source; check refreshed releases and the sign-in guidance.
2. Run xcode-select in Terminal, then return to Xcodes; check the active selection updates.
3. Fail a helper-backed selection and check Settings/Copy Command recovery. Fail post-install setup and check Open Xcode recovery.

Consolidates #863, #864, #866, #867 and the related maintenance from #869. Related independent topics: UI update and Platform management. The combined preview branch contains the complete app and tested resolutions for the shared code.

Related review topics: #873, #875.

Related issues

• Got an error message that says unable to install xCode. not much explanation #616 — gives helper-backed installation failures and Apple data-source authentication failures a specific explanation and recovery action.
• Ask admin credentials if Xcode installation in /Applications is not allowed #112 — when a privileged file operation fails, offers Advanced Settings or a copyable Terminal command. This is a recovery path, not a replacement for managed administrator authorization.