Adds a Favorites feature: a heart on the offer detail saves another
person's listing to an encrypted, per-identity SavedOffersStore
(keystore JSON snapshot, no plaintext at rest). A new /favorites screen
(wired from the drawer) lists saved offers offline-first and, when a
relay is reachable, flags ones that are gone as "no longer available".
i18n en/es/pt/ast; store + screen + detail-heart tests.
