accounts ui: Add "About Zulip" page, reachable from "Choose account" page

From this new page, the user can tap a button to show [LicensePage],
so resolving #99.

Fixes: #99

Author: chrisbobbe

lib/widgets/about_zulip.dart

@@ -0,0 +1,56 @@
+import 'package:flutter/material.dart';
+import 'package:package_info_plus/package_info_plus.dart';
+
+class AboutZulipPage extends StatefulWidget {
+  const AboutZulipPage({super.key});
+
+  static Route<void> buildRoute(BuildContext context) {
+    return MaterialPageRoute(builder: (context) => const AboutZulipPage());
+  }
+
+  @override
+  State<AboutZulipPage> createState() => _AboutZulipPageState();
+}
+
+class _AboutZulipPageState extends State<AboutZulipPage> {
+  PackageInfo? _packageInfo;
+
+  @override
+  void initState() {
+    super.initState();
+    (() async {
+      final result = await PackageInfo.fromPlatform();
+      setState(() {
+        _packageInfo = result;
+      });
+    })();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text("About Zulip")),
+      body: SingleChildScrollView(
+        child: SafeArea(
+          minimum: const EdgeInsets.all(8), // ListView pads vertical
+          child: Center(
+            child: ConstrainedBox(
+              constraints: const BoxConstraints(maxWidth: 400),
+              child: Column(mainAxisAlignment: MainAxisAlignment.center, children: [
+                ListTile(
+                  title: const Text('App version'),
+                  subtitle: Text(_packageInfo?.version ?? '(…)')),
+                ListTile(
+                  title: const Text('Open-source licenses'),
+                  subtitle: const Text('Tap to view'),
+                  onTap: () {
+                    // TODO(upstream?): This route and its child routes (pushed
+                    //   when you tap a package to view its licenses) can't be
+                    //   popped on iOS with the swipe-away gesture; you have to
+                    //   tap the "Back" button. Debug/fix.
+                    showLicensePage(context: context);
+                  }),
+              ])))),
+      ));
+  }
+}

lib/widgets/app.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 
+import 'about_zulip.dart';
 import 'compose_box.dart';
 import 'login.dart';
 import 'message_list.dart';
@@ -57,7 +58,9 @@ class ChooseAccountPage extends StatelessWidget {
     assert(!PerAccountStoreWidget.debugExistsOf(context));
     final globalStore = GlobalStoreWidget.of(context);
     return Scaffold(
-      appBar: AppBar(title: const Text('Choose account')),
+      appBar: AppBar(
+        title: const Text('Choose account'),
+        actions: const [ChooseAccountPageOverflowButton()]),
       body: SafeArea(
         minimum: const EdgeInsets.all(8),
         child: Center(
@@ -79,6 +82,28 @@ class ChooseAccountPage extends StatelessWidget {
   }
 }
 
+enum ChooseAccountPageOverflowMenuItem { aboutZulip }
+
+class ChooseAccountPageOverflowButton extends StatelessWidget {
+  const ChooseAccountPageOverflowButton({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return PopupMenuButton<ChooseAccountPageOverflowMenuItem>(
+      itemBuilder: (BuildContext context) => const [
+        PopupMenuItem(
+          value: ChooseAccountPageOverflowMenuItem.aboutZulip,
+          child: Text('About Zulip')),
+      ],
+      onSelected: (item) {
+        switch (item) {
+          case ChooseAccountPageOverflowMenuItem.aboutZulip:
+            Navigator.push(context, AboutZulipPage.buildRoute(context));
+        }
+      });
+  }
+}
+
 class HomePage extends StatelessWidget {
   const HomePage({super.key});