fix(menubar): clarify update failure status

[ZacharyHu0]

Summary

• replace the ambiguous "Failed" badge with stage-specific labels:
  • "Update Check Failed"
  • "CLI Update Failed"
  • "Menubar Update Failed"
• show the failure stage, underlying error, and retry action on hover
• retry the update check directly when the automatic check itself failed
• add tests for each failure presentation state

Addresses #806

Testing

• Swift type checking passed for the update-checker sources
• Swift syntax validation passed for the changed view and tests
• Full swift test could not run because the local Command Line Tools installation is missing the SwiftUIMacros plugin required by existing SwiftUI files

[iamtoruk]

Merged — thanks for a well-shaped first contribution: issue-with-screenshot first, surgical scope, complete stage coverage across every failure site (audited), and tests pinning the user-facing copy. We ran the full Swift suite on your behalf (120 green merged with main). The clearer badges ship with the next menubar release.