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/ui/restart_widget.dart

@@ -0,0 +1,30 @@
+import 'package:flutter/widgets.dart';
+
+/// Rebuilds its whole subtree from scratch on demand — used to apply a social
+/// identity switch, which re-registers the social singletons in the locator and
+/// needs the widget tree (router, screens, sessions) rebuilt to pick them up.
+///
+/// [builder] is re-invoked on each restart, so it should read its dependencies
+/// fresh (e.g. from the service locator) rather than close over stale ones.
+class RestartWidget extends StatefulWidget {
+  const RestartWidget({required this.builder, super.key});
+
+  final Widget Function() builder;
+
+  /// Tears down and rebuilds everything under the nearest [RestartWidget].
+  static void restart(BuildContext context) =>
+      context.findAncestorStateOfType<_RestartWidgetState>()?.restart();
+
+  @override
+  State<RestartWidget> createState() => _RestartWidgetState();
+}
+
+class _RestartWidgetState extends State<RestartWidget> {
+  Key _key = UniqueKey();
+
+  void restart() => setState(() => _key = UniqueKey());
+
+  @override
+  Widget build(BuildContext context) =>
+      KeyedSubtree(key: _key, child: widget.builder());
+}