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/main.dart

@@ -2,6 +2,7 @@ import 'package:flutter/widgets.dart';
 
 import 'bootstrap.dart';
 import 'i18n/strings.g.dart';
+import 'ui/restart_widget.dart';
 
 void main() {
   WidgetsFlutterBinding.ensureInitialized();
@@ -10,6 +11,12 @@ void main() {
   // (e.g. Asturian). The saved choice is restored inside [Bootstrap], after DI.
   LocaleSettings.useDeviceLocaleSync();
   // Paint immediately: [Bootstrap] shows a splash and runs the heavy init off
-  // the first frame, so the app window appears at once instead of after DI.
-  runApp(TranslationProvider(child: const Bootstrap()));
+  // the first frame. Wrapped in [RestartWidget] so switching the social identity
+  // (which re-registers the social singletons) can rebuild the whole tree —
+  // Bootstrap re-reads its dependencies from the locator on each rebuild.
+  runApp(
+    RestartWidget(
+      builder: () => TranslationProvider(child: const Bootstrap()),
+    ),
+  );
 }