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/app.dart

@@ -16,6 +16,7 @@ import 'services/offer_outbox.dart';
 import 'services/onboarding_store.dart';
 import 'services/profile_cache.dart';
 import 'services/profile_store.dart';
+import 'services/social_account_store.dart';
 import 'services/social_service.dart';
 import 'services/social_settings.dart';
 import 'state/inventory_cubit.dart';
@@ -50,6 +51,7 @@ class TaneApp extends StatelessWidget {
     this.messageStore,
     this.profileStore,
     this.profileCache,
+    this.socialAccounts,
     this.inbox,
     this.showIntro = false,
     this.autoBackup,
@@ -65,6 +67,7 @@ class TaneApp extends StatelessWidget {
           messageStore,
           profileStore,
           profileCache,
+          socialAccounts,
           inbox,
         );
 
@@ -92,6 +95,9 @@ class TaneApp extends StatelessWidget {
   /// Optional cache of peers' published display names.
   final ProfileCache? profileCache;
 
+  /// Optional store of the active social identity, for the profile switcher.
+  final SocialAccountStore? socialAccounts;
+
   /// App-wide inbox listener; drives the messages list's live refresh.
   final InboxService? inbox;
   final bool showIntro;
@@ -112,6 +118,7 @@ class TaneApp extends StatelessWidget {
     MessageStore? messageStore,
     ProfileStore? profileStore,
     ProfileCache? profileCache,
+    SocialAccountStore? socialAccounts,
     InboxService? inbox,
   ) {
     return GoRouter(
@@ -157,13 +164,17 @@ class TaneApp extends StatelessWidget {
               inbox: inbox,
             ),
           ),
-        if (social != null && socialSettings != null && profileStore != null)
+        if (social != null &&
+            socialSettings != null &&
+            profileStore != null &&
+            socialAccounts != null)
           GoRoute(
             path: '/profile',
             builder: (context, state) => ProfileScreen(
               social: social,
               settings: socialSettings,
               profileStore: profileStore,
+              accounts: socialAccounts,
             ),
           ),
         if (social != null && socialSettings != null)