Add JSON backup and restore

[SamDozer]

Smaller than cloud sync and useful immediately: let people get their practice
history off the phone and onto a new one.

Uninstalling currently deletes everything permanently. For anyone with hundreds
of hours logged, that is the scariest property of the app.

• Export everything to a single JSON file
• Import it back, on the same phone or a new one
• Import into a non-empty database without duplicating
• Version the format so a future release can still read today's backup

No server involved. Shares design work with #3 — a backup is roughly "export
everything" rather than "export one routine".