fix(ui): skip invalid MacOSPref entries (empty domain/key) in snapshot editor

[fullstackjam]

Summary

• A corrupted or old-format snapshot file can contain a MacOSPref entry with an empty Key (e.g. Domain="cirruslabs/cli", Key=""), which caused that entry to appear in the macOS Prefs tab as "cirruslabs/cli." — visually indistinguishable from just "cirruslabs/cli" because the trailing dot is tiny and the "= ()" description renders in faint gray.
• Every valid macOS preference requires both a non-empty Domain and a non-empty Key. Entries missing either are now skipped when building the macOS Prefs tab in NewSnapshotEditor.
• The tap itself (cirruslabs/cli) is already correctly shown in the Taps tab from snap.Packages.Taps; only the phantom MacOSPref entry is dropped.

What changed

• internal/ui/snapshot_editor.go: Changed pref-items loop from pre-allocated slice to append-with-guard, skipping any MacOSPref where Domain == "" or Key == "".
• internal/ui/snapshot_editor_test.go: Added TestNewSnapshotEditorSkipsInvalidMacOSPrefs to cover tap-misclassified-as-pref entries, empty-domain entries, and valid new prefs.

Test plan

• make test-unit passes
• Run openboot snapshot capture and confirm no items appear in the macOS Prefs tab that look like Homebrew taps
• Import a snapshot file containing a macos_prefs entry with empty key and confirm it is silently dropped from the macOS Prefs tab

[github-actions]

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Before merging:

• Code follows existing patterns in the codebase
• go build ./... and go vet ./... pass
• Commit message is clear and descriptive

@fullstackjam will review this soon. Thanks for contributing! 🚀

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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