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/i18n/pt.i18n.json

@@ -414,7 +414,15 @@
     "copy": "Copiar",
     "copied": "Copiado",
     "save": "Guardar",
-    "saved": "Perfil guardado"
+    "saved": "Perfil guardado",
+    "identities": "As tuas identidades",
+    "identitiesHelp": "Mantém identidades separadas — cada uma com as suas mensagens e contactos. Todas vêm da tua única cópia de segurança, por isso mudar não acrescenta nada para lembrar.",
+    "identityLabel": "Identidade {n}",
+    "current": "Em uso",
+    "newIdentity": "Nova identidade",
+    "switchTitle": "Mudar de identidade?",
+    "switchBody": "As mensagens e contactos são guardados separadamente para cada identidade. Podes voltar quando quiseres.",
+    "switchAction": "Mudar"
   },
   "chatList": {
     "title": "Mensagens",