fix(hotkey): make shortcut changes replace immediately

[dschnurbusch]

Summary

• make Change persist and push the replacement shortcut directly to the live global hotkey manager instead of relying on SwiftUI onChange
• route shortcut removal through the same live-update path
• allow removing the final primary dictation shortcut and persist the explicit unbound state
• add regression coverage for clearing all primary dictation shortcuts

Verification

• xcrun swiftc -parse on all four changed Swift files
• git diff --check
• full Xcode build/tests run in GitHub Actions

This fork PR exists to run the repository's full macOS CI before opening the upstream PR.

[dschnurbusch]

Closing the fork-only CI probe; the upstream PR is altic-dev#707.

[dschnurbusch]

Fork CI passed; upstream review continues at altic-dev#707.