accounts: Add global "Help" menu item, with option to show [AboutDialog]

From Flutter's [AboutDialog], the user can tap a button to show
[LicensePage], so resolving #99.

Fixes: #99

Author: chrisbobbe

lib/widgets/app.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import 'compose_box.dart';
+import 'draggable_scrollable_modal_bottom_sheet.dart';
 import 'login.dart';
 import 'message_list.dart';
 import 'page.dart';
@@ -82,18 +83,43 @@ class ChooseAccountPage extends StatelessWidget {
         SafeArea(
           minimum: const EdgeInsets.all(8),
           child: Column(children: [
-            MenuItemButton(
-              // round corners; not flush with device edge (SafeArea padding)
-              style: const ButtonStyle(shape: MaterialStatePropertyAll(StadiumBorder())),
+            _BottomMenuButton(
               leadingIcon: const Icon(Icons.add),
               onPressed: () => Navigator.push(context,
                 AddAccountPage.buildRoute()),
               child: const Text('Add an account')),
+            _BottomMenuButton(
+              leadingIcon: const Icon(Icons.help_outline),
+              onPressed: () {
+                // Could make a whole page for global help.
+                showDraggableScrollableModalBottomSheet(context: context,
+                  builder: (context) => Column(children: [
+                    // TODO button for https://zulip.com/help/
+                    MenuItemButton(
+                      onPressed: () {
+                        // TODO: Fill in [AboutDialog] data: app version, icon, etc:
+                        //   https://api.flutter.dev/flutter/material/AboutDialog/AboutDialog.html
+                        showAboutDialog(context: context);
+                      },
+                      child: const Text('About Zulip')),
+                  ]));
+              },
+              child: const Text('Help')),
           ])),
       ]));
   }
 }
 
+class _BottomMenuButton extends MenuItemButton {
+  const _BottomMenuButton({
+    required super.onPressed,
+    required super.leadingIcon,
+    required super.child,
+  }) : super(
+    // round corners; not flush with device edge (SafeArea padding)
+    style: const ButtonStyle(shape: MaterialStatePropertyAll(StadiumBorder())));
+}
+
 class HomePage extends StatelessWidget {
   const HomePage({super.key});