icon: Add icon for switch account, from Figma

taken from Figma:
	https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6141-40422&t

Author: rishichirchi

assets/icons/arrow_left_right.svg

@@ -553,11 +553,8 @@ class _SwitchAccountButton extends _MenuButton {
   const _SwitchAccountButton();
 
   @override
-  // TODO(design): choose an icon
-  IconData? get icon => null;
-
-  @override
-  Widget buildLeading(BuildContext context) => const SizedBox.shrink();
+ 
+  IconData? get icon => ZulipIcons.arrow_left_right;
 
   @override
   String label(ZulipLocalizations zulipLocalizations) {

lib/widgets/home.dart

@@ -27,17 +27,20 @@ abstract final class ZulipIcons {
   /// The Zulip custom icon "arrow_down".
   static const IconData arrow_down = IconData(0xf101, fontFamily: "Zulip Icons");
 
+  /// The Zulip custom icon "arrow_left_right".
+  static const IconData arrow_left_right = IconData(0xf102, fontFamily: "Zulip Icons");
+
   /// The Zulip custom icon "arrow_right".
-  static const IconData arrow_right = IconData(0xf102, fontFamily: "Zulip Icons");
+  static const IconData arrow_right = IconData(0xf103, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "at_sign".
-  static const IconData at_sign = IconData(0xf103, fontFamily: "Zulip Icons");
+  static const IconData at_sign = IconData(0xf104, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "attach_file".
-  static const IconData attach_file = IconData(0xf104, fontFamily: "Zulip Icons");
+  static const IconData attach_file = IconData(0xf105, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "bot".
-  static const IconData bot = IconData(0xf105, fontFamily: "Zulip Icons");
+  static const IconData bot = IconData(0xf106, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "camera".
   static const IconData camera = IconData(0xf106, fontFamily: "Zulip Icons");