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/message_store.dart

@@ -23,9 +23,13 @@ class ChatSummary {
 }
 
 class MessageStore {
-  MessageStore(this._store);
+  /// [accountScope] namespaces the keys per social identity (empty = the
+  /// original identity's legacy keys). See [socialAccountScope].
+  MessageStore(this._store, {String accountScope = ''})
+      : _base = accountScope.isEmpty ? 'tane.social.' : 'tane.social.$accountScope.';
 
   final SecretStore _store;
+  final String _base;
 
   /// Serializes [append]'s read-modify-write. Several sources now write the same
   /// conversation concurrently (the app-wide inbox listener and an open chat's
@@ -33,17 +37,15 @@ class MessageStore {
   /// read the same history and the second write would clobber the first.
   Future<void> _writeTail = Future.value();
 
-  static const _prefix = 'tane.social.chat.';
-
   /// Index of peers we have a conversation with (the keystore is key/value with
   /// no key enumeration, so we track the list ourselves).
-  static const _indexKey = 'tane.social.chats';
+  String get _indexKey => '${_base}chats';
 
   /// Keep only the most recent [_cap] messages per conversation, to bound the
   /// keystore entry size.
   static const _cap = 200;
 
-  String _key(String peerPubkey) => '$_prefix$peerPubkey';
+  String _key(String peerPubkey) => '${_base}chat.$peerPubkey';
 
   /// Messages exchanged with [peerPubkey], oldest first.
   Future<List<PrivateMessage>> history(String peerPubkey) async {