Add account switcher

Fixes #146

lib/helpers/extensions/log_out.dart

@@ -0,0 +1,53 @@
+import "package:collection/collection.dart";
+import "package:flutter_riverpod/experimental/mutation.dart";
+import "package:nexus/controllers/accounts.dart";
+import "package:nexus/controllers/active_account.dart";
+import "package:nexus/controllers/client.dart";
+import "package:nexus/controllers/init_complete.dart";
+import "package:nexus/controllers/unified_push.dart";
+import "package:nexus/main.dart";
+
+extension LogOut on MutationTarget {
+  Future<void> logOut(String accountId) async {
+    final accounts = await container.read(AccountsController.provider.future);
+    final account = accounts.firstWhereOrNull(
+      (account) => account.id == accountId,
+    );
+    if (account == null) return;
+
+    final isActive =
+        accountId == container.read(ActiveAccountController.provider);
+    final client = isActive
+        ? container.read(ClientController.provider.notifier)
+        : container.read(ClientController.forAccount(accountId).notifier);
+
+    if (account.deviceId case final deviceId?) {
+      await container
+          .read(UnifiedPushController.provider.notifier)
+          .deregister(deviceId)
+          .onError(showError);
+    }
+    if (isActive) container.invalidate(InitCompleteController.provider);
+    await client.logout().onError(showError);
+
+    if (!isActive) {
+      container.invalidate(ClientController.forAccount(accountId));
+      await container
+          .read(AccountsController.provider.notifier)
+          .remove(accountId);
+      return;
+    }
+
+    final other = accounts.firstWhereOrNull(
+      (account) => account.id != accountId,
+    );
+    if (other == null) return;
+
+    await container
+        .read(AccountsController.provider.notifier)
+        .remove(accountId);
+    await container
+        .read(ActiveAccountController.provider.notifier)
+        .switchTo(other.id);
+  }
+}