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/test/services/message_store_test.dart

@@ -59,6 +59,21 @@ void main() {
     expect(await store.history('peer'), hasLength(2));
   });
 
+  test('per-identity scope isolates conversations (0 = legacy keys)', () async {
+    final secret = InMemorySecretStore();
+    final acct0 = MessageStore(secret); // legacy / account 0
+    final acct1 = MessageStore(secret, accountScope: 'acct1');
+
+    await acct0.append('peer', msg('peer', 'for identity 0', 1000));
+    await acct1.append('peer', msg('peer', 'for identity 1', 2000));
+
+    // Same peer, but each identity sees only its own conversation.
+    expect((await acct0.history('peer')).single.text, 'for identity 0');
+    expect((await acct1.history('peer')).single.text, 'for identity 1');
+    expect((await acct0.conversations()).single.lastText, 'for identity 0');
+    expect((await acct1.conversations()).single.lastText, 'for identity 1');
+  });
+
   test('history is capped to the most recent 200', () async {
     for (var i = 0; i < 210; i++) {
       await store.append('peer', msg('me', 'm$i', i));