A new Backup & Restore card saves everything the app persists — songs, presets
(with their sample WAVs), themes, the autosaved project, and the settings +
input-binding DataStore blobs — into one .zip via the system file picker, and
restores from one.
Restore is defensive: it extracts to a staging folder and only swaps it into
place once the archive validates, so a corrupt/wrong file can't destroy current
data; it's zip-slip guarded and runs off the main thread. Since DataStore caches
in memory, restore prompts to reopen the app to finish applying. Covered by
BackupIoTest (round-trip replace, bad-file safety, empty-zip rejection).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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