Add account switcher

Fixes #146

lib/models/account.dart

@@ -0,0 +1,26 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+
+part "account.freezed.dart";
+part "account.g.dart";
+
+/// An account slot, each with its own gomuks directories.
+///
+/// Everything but [id] is cached from the last time the account was active, so
+/// the account switcher and push routing work while it isn't.
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const Account({
+  required final String id,
+  final String? userId,
+  final String? deviceId,
+  final String? displayName,
+  final Uri? avatar,
+}) with _$Account {
+  /// The slot accounts from before account switching existed
+  static const String legacyId = "default";
+
+  Map<String, Object?> toJson() => _$AccountToJson(this);
+
+  factory Account.fromJson(Map<String, Object?> json) =>
+      _$AccountFromJson(json);
+}