Persistence + wiring the offers screen needs, per the product decisions:
- SocialSettings (keystore-backed, no plaintext): coarse area geohash +
configurable relay URLs, both empty by default (no bundled public relays —
don't leak metadata to third parties). Registered in DI.
- createOffersCubit(): opens a SocialSession on the first configured relay when
online, else a null-transport cubit that degrades gracefully. OffersCubit now
closes the owning session on dispose.
4 settings tests green; offers/social suites still green.
