feat(backup): back up and restore the encrypted vault from the menu

[pyramation]

Summary

The vault is already a single sealed file, so backup is a copy — no new format, no second key. This adds File → Back Up Vault… (⌘⇧B) and Restore from Backup…, mirrored as buttons in Settings → Backup, with copy that spells out that the copy is encrypted and only the master password opens it (so putting it in iCloud Drive/OneDrive/Dropbox is safe).

The app had no menu at all before (autoHideMenuBar: true + Electron's default template), so menu.ts adds a real one; the menu is now always visible on Windows/Linux since it carries the backup items.

Three things worth calling out in the implementation:

// service.flush(): the debounced save is up to 2s behind the UI, so a backup
// taken right after an edit would silently miss it
await service.flush();          // cancels the timer, awaits any in-flight lock, saves
await fs.copyFile(vaultFilePath(), chosen);

// restore validates before it destroys anything — parseHeader throws
// CorruptEnvelopeError on a non-envelope, so a stray .jpg can't replace a vault
await assertEnvelope(chosen);
await service.lock();
const kept = `${dir}/replaced-${backupName()}`;
await fs.rename(target, kept);               // recoverable if the user picked wrong
try { await fs.copyFile(chosen, target); }
catch (err) { await fs.rename(kept, target); throw err; }   // never leave no vault

Restore fires lockedEvent so the renderer drops to the unlock screen — the restored file needs the password it was made with, which may not be the current one.

totp.code's TotpEntry | null from #15 is unchanged here; the new IPC surface is backup.create/restore/revealVault.

Link to Devin session: https://app.devin.ai/sessions/04636534e07048089ffb6b78142e12cd
Requested by: @pyramation

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