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/en.i18n.json

@@ -418,7 +418,15 @@
     "copy": "Copy",
     "copied": "Copied",
     "save": "Save",
-    "saved": "Profile saved"
+    "saved": "Profile saved",
+    "identities": "Your identities",
+    "identitiesHelp": "Keep separate identities — each with its own messages and contacts. They all come from your one backup, so switching adds nothing to remember.",
+    "identityLabel": "Identity {n}",
+    "current": "In use",
+    "newIdentity": "New identity",
+    "switchTitle": "Switch identity?",
+    "switchBody": "Messages and contacts are kept separate for each identity. You can switch back anytime.",
+    "switchAction": "Switch"
   },
   "chatList": {
     "title": "Messages",