login: Link to doc for what "server URL" is and how to find it

assets/l10n/app_en.arb

@@ -289,6 +289,10 @@
   "@loginServerUrlInputLabel": {
     "description": "Input label in login page for Zulip server URL entry."
   },
+  "serverURLDocLinkLabel": "(What's this?)",
+  "@serverURLDocLinkLabel": {
+    "description": "Link to doc to help users understand what a server URL is and how to find theirs."
+  },
   "loginHidePassword": "Hide password",
   "@loginHidePassword": {
     "description": "Icon label for button to hide password in input form."

lib/widgets/login.dart

@@ -1,10 +1,12 @@
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 
 import '../api/exception.dart';
 import '../api/route/account.dart';
 import '../api/route/realm.dart';
 import '../api/route/users.dart';
+import '../model/binding.dart';
 import '../model/store.dart';
 import 'app.dart';
 import 'dialog.dart';
@@ -223,7 +225,13 @@ class _AddAccountPageState extends State<AddAccountPage> {
                 decoration: InputDecoration(
                   labelText: zulipLocalizations.loginServerUrlInputLabel,
                   errorText: errorText,
-                  helperText: kLayoutPinningHelperText,
+                  helper: InkWell(
+                    onTap: () {
+                      _launchUrl(context);
+                    },
+                    child: Text(
+                      zulipLocalizations.serverURLDocLinkLabel,
+                      style: const TextStyle(color: Colors.blue))),
                   hintText: 'your-org.zulipchat.com')),
               const SizedBox(height: 8),
               ElevatedButton(
@@ -235,6 +243,30 @@ class _AddAccountPageState extends State<AddAccountPage> {
   }
 }
 
+void _launchUrl(BuildContext context) async {
+  String urlString = 'https://zulip.com/help/logging-in#find-the-zulip-log-in-url';
+  Future<void> showError(BuildContext context, String? message) {
+    return showErrorDialog(context: context,
+      title: 'Unable to open link',
+      message: [
+        'Link could not be opened: $urlString',
+        if (message != null) message,
+      ].join("\n\n"));
+  }
+
+  bool launched = false;
+  String? errorMessage;
+  try {
+    launched = await ZulipBinding.instance.launchUrl(Uri.parse(urlString));
+  } on PlatformException catch (e) {
+    errorMessage = e.message;
+  }
+  if (!launched) {
+    if (!context.mounted) return;
+    await showError(context, errorMessage);
+  }
+}
+
 class PasswordLoginPage extends StatefulWidget {
   const PasswordLoginPage({super.key, required this.serverSettings});