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/services/profile_cache.dart

@@ -4,10 +4,15 @@ import '../security/secret_store.dart';
 /// `name`), so the inbox and chat show a human name instead of a raw key —
 /// even offline. Keystore-backed (no plaintext at rest).
 class ProfileCache {
-  ProfileCache(this._store);
+  /// [accountScope] namespaces the keys per social identity (empty = the
+  /// original identity's legacy keys). See [socialAccountScope].
+  ProfileCache(this._store, {String accountScope = ''})
+      : _prefix = accountScope.isEmpty
+            ? 'tane.social.name.'
+            : 'tane.social.$accountScope.name.';
 
   final SecretStore _store;
-  static const _prefix = 'tane.social.name.';
+  final String _prefix;
 
   /// The cached name for [pubkeyHex], or null if none is known.
   Future<String?> name(String pubkeyHex) async {