bug: subscriptions and pending payment proofs are lost after a wallet restore #812

Refs:

• fix: preserve paykit payment state #786
• fix: preserve paykit payment state bitkit-android#1334

What happened?

Wallet backups don't include accepted subscriptions or pending payment proofs, and unreadable Paykit state is treated as empty, so a restore can lose them.

Expected behavior

A restored wallet keeps its accepted subscriptions and pending payment proofs, and unreadable Paykit state is preserved instead of deleted.