feat(backup): export any backup version as JSON

[dmnyc]

Stacked on #641 — this branch is cut from feat/settings-follow-recovery, so it also contains that commit. Both touch NostrBackupSection.svelte; keeping them stacked avoids a guaranteed conflict. Merge #641 first and this diff reduces to the export commit alone.

Why

Backups are NIP-78 events only the user can decrypt. That's right for privacy and wrong for portability: the data could only ever be read back by this app, and only for as long as the account's signer still works. There was no way to get a copy out.

The change

"Export JSON" on every backup row — Follows, Mute List, Profile — downloading the latest backup's contents as a file.

The expandable Versions list gets a per-version JSON button too, next to its existing Restore. That's arguably the more useful one: when several versions exist, "export this one" is the thing you actually want.

listFollowsBackups() and friends already decrypt eagerly, so the payload is in hand — export costs no extra signer round-trip and no network call.

File contents

The payload is wrapped in a little provenance so the file is still intelligible months later, out of context:

{
  "source": "zap.cooking",
  "exportedAt": "2026-08-20T…",
  "pubkey": "…",
  "type": "follows",
  "kind": "kind:3",
  "backedUpAt": "2026-08-14T…",
  "data": { … }
}

Filename is zapcooking-<type>-<backup date>.json — dated by when the backup was taken, not when it was exported, so several exports of different versions don't collide.

Buttons are disabled when a row has no decrypted backup to export.

Verification

• pnpm check — 1 error / 152 warnings, identical to main (pre-existing delete-operator error)
• pnpm test — 1267 passed
• Checked in a browser that all three rows render an Export JSON button, correctly disabled when there is nothing to export

Not verified: an actual download with real backup data, which needs a signed-in account with backups. The download path is the standard blob/anchor approach, but the resulting file is worth eyeballing once.

Worth deciding separately

There's no import counterpart. Export alone is still worth having — it gets data out, which is the part that's currently impossible — but "restore from a JSON file" is the obvious follow-up if you want these files to be more than an archive.