page [nfc]: Add interface to get account ID for most of our routes

lib/widgets/home.dart

@@ -31,7 +31,7 @@ enum _HomePageTab {
 class HomePage extends StatefulWidget {
   const HomePage({super.key});
 
-  static Route<void> buildRoute({required int accountId}) {
+  static AccountRoute<void> buildRoute({required int accountId}) {
     return MaterialAccountWidgetRoute(accountId: accountId,
       loadingPlaceholderPage: _LoadingPlaceholderPage(accountId: accountId),
       page: const HomePage());

lib/widgets/message_list.dart

@@ -188,7 +188,7 @@ abstract class MessageListPageState {
 class MessageListPage extends StatefulWidget {
   const MessageListPage({super.key, required this.initNarrow});
 
-  static Route<void> buildRoute({int? accountId, BuildContext? context,
+  static AccountRoute<void> buildRoute({int? accountId, BuildContext? context,
       required Narrow narrow}) {
     return MaterialAccountWidgetRoute(accountId: accountId, context: context,
       page: MessageListPage(initNarrow: narrow));

lib/widgets/page.dart

@@ -11,6 +11,12 @@ abstract class WidgetRoute<T extends Object?> extends PageRoute<T> {
   Widget get page;
 }
 
+/// A page route that specifies a particular Zulip account to use, by ID.
+abstract class AccountRoute<T extends Object?> extends PageRoute<T> {
+  /// The [Account.id] of the account to use for this page.
+  int get accountId;
+}
+
 /// A [MaterialPageRoute] that always builds the same widget.
 ///
 /// This is useful for making the route more transparent for a test to inspect.
@@ -32,8 +38,10 @@ class MaterialWidgetRoute<T extends Object?> extends MaterialPageRoute<T> implem
 }
 
 /// A mixin for providing a given account's per-account store on a page route.
-mixin AccountPageRouteMixin<T extends Object?> on PageRoute<T> {
+mixin AccountPageRouteMixin<T extends Object?> on PageRoute<T> implements AccountRoute<T> {
+  @override
   int get accountId;
+
   Widget? get loadingPlaceholderPage;
 
   @override

lib/widgets/profile.dart

@@ -30,7 +30,7 @@ class ProfilePage extends StatelessWidget {
 
   final int userId;
 
-  static Route<void> buildRoute({int? accountId, BuildContext? context,
+  static AccountRoute<void> buildRoute({int? accountId, BuildContext? context,
       required int userId}) {
     return MaterialAccountWidgetRoute(accountId: accountId, context: context,
       page: ProfilePage(userId: userId));

test/widgets/page_checks.dart

@@ -6,6 +6,6 @@ extension WidgetRouteChecks<T> on Subject<WidgetRoute<T>> {
   Subject<Widget> get page => has((x) => x.page, 'page');
 }
 
-extension AccountPageRouteMixinChecks<T> on Subject<AccountPageRouteMixin<T>> {
+extension AccountRouteChecks<T> on Subject<AccountRoute<T>> {
   Subject<int> get accountId => has((x) => x.accountId, 'accountId');
 }