nav: Go straight to an account's home page on launch, when available

lib/widgets/app.dart

@@ -112,6 +112,9 @@ class ZulipApp extends StatelessWidget {
 
     return GlobalStoreWidget(
       child: Builder(builder: (context) {
+        final globalStore = GlobalStoreWidget.of(context);
+        // TODO(#524) choose initial account as last one used
+        final initialAccountId = globalStore.accounts.firstOrNull?.id;
         return MaterialApp(
           title: 'Zulip',
           localizationsDelegates: ZulipLocalizations.localizationsDelegates,
@@ -141,6 +144,10 @@ class ZulipApp extends StatelessWidget {
           onGenerateInitialRoutes: (_) {
             return [
               MaterialWidgetRoute(page: const ChooseAccountPage()),
+              if (initialAccountId != null) ...[
+                MaterialAccountWidgetRoute(accountId: initialAccountId,
+                  page: const HomePage()),
+              ],
             ];
           });
         }));

test/widgets/app_test.dart

@@ -4,6 +4,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:zulip/widgets/app.dart';
 import 'package:zulip/widgets/page.dart';
 
+import '../example_data.dart' as eg;
 import '../model/binding.dart';
 import '../test_navigation.dart';
 import 'page_checks.dart';
@@ -23,11 +24,27 @@ void main() {
       return pushedRoutes;
     }
 
-    testWidgets('go to choose-account page', (tester) async {
+    testWidgets('when no accounts, go to choose account', (tester) async {
       addTearDown(testBinding.reset);
       check(await initialRoutes(tester)).deepEquals([
         (Subject it) => it.isA<WidgetRoute>().page.isA<ChooseAccountPage>(),
       ]);
     });
+
+    testWidgets('when have accounts, go to home page for first account', (tester) async {
+      addTearDown(testBinding.reset);
+
+      // We'll need per-account data for the account that a page will be opened
+      // for, but not for the other account.
+      await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
+      await testBinding.globalStore.insertAccount(eg.otherAccount.toCompanion(false));
+
+      check(await initialRoutes(tester)).deepEquals([
+        (Subject it) => it.isA<WidgetRoute>().page.isA<ChooseAccountPage>(),
+        (Subject it) => it.isA<MaterialAccountWidgetRoute>()
+          ..accountId.equals(eg.selfAccount.id)
+          ..page.isA<HomePage>(),
+      ]);
+    });
   });
 }

test/widgets/page_checks.dart

@@ -5,3 +5,7 @@ import 'package:zulip/widgets/page.dart';
 extension WidgetRouteChecks on Subject<WidgetRoute> {
   Subject<Widget> get page => has((x) => x.page, 'page');
 }
+
+extension AccountPageRouteMixinChecks on Subject<AccountPageRouteMixin> {
+  Subject<int> get accountId => has((x) => x.accountId, 'accountId');
+}