feat(identity): switch social identity, all from the one backup

Adds pseudonymous, switchable social identities derived from the SAME root
seed via an account index (NostrKeyDerivation.deriveFromSeed(seed, account)).
HKDF is one-way so accounts are unlinkable to the Ğ1 key; account 0 is the
original identity, byte-for-byte unchanged (no rotation for current users),
and every account regenerates from the single seed — so switching adds
nothing to back up.

- SocialAccountStore: keystore-backed active account + max created.
- Per-identity stores (chats, profile, name cache) namespaced by account
  scope (account 0 = legacy keys, no migration) so identities never mix.
- switchSocialAccount() re-derives the identity, re-scopes the stores and
  restarts the inbox listener; RestartWidget rebuilds the tree to pick up
  the new social singletons. DB/inventory untouched.
- Profile 'Your identities' switcher: list, create, switch (with a note
  that messages/contacts are kept separate per identity). i18n en/es/pt/ast.

Tests: account-indexed derivation (legacy 0 unchanged, accounts distinct
yet deterministic, negatives rejected); SocialAccountStore; per-identity
store scope isolation. Resolves the flagged 'change identity' decision
(open-decisions §B).

Known: switching resets navigation to home (full tree rebuild).

apps/app_seeds/lib/bootstrap.dart

@@ -17,6 +17,7 @@ import 'services/offer_outbox.dart';
 import 'services/onboarding_store.dart';
 import 'services/profile_cache.dart';
 import 'services/profile_store.dart';
+import 'services/social_account_store.dart';
 import 'services/social_service.dart';
 import 'services/social_settings.dart';
 import 'ui/theme.dart';
@@ -67,6 +68,7 @@ class _BootstrapState extends State<Bootstrap> {
       messageStore: getIt<MessageStore>(),
       profileStore: getIt<ProfileStore>(),
       profileCache: getIt<ProfileCache>(),
+      socialAccounts: getIt<SocialAccountStore>(),
       inbox: inbox,
       showIntro: !await onboarding.introSeen(),
       autoBackup: getIt.isRegistered<AutoBackupService>()