button: Implement a disabled state for Toggle

Vlad suggested using 0.5 opacity if no special disabled state is
provided:
  https://chat.zulip.org/#narrow/channel/530-mobile-design/topic/toggle.3A.20disabled.20state/near/2250883
but Greg and I prefer 0.4 in this case to make it more distinct.

We may or may not end up using this for unsubscribed channels
without content access (i.e. ones that you can't subscribe yourself
to). We may instead just not show a toggle for those channels.

Author: chrisbobbe

lib/widgets/button.dart

@@ -477,6 +477,9 @@ enum ZulipMenuItemButtonStyle {
 
 /// The "toggle" component in Figma.
 ///
+/// If [onChanged] is null, the switch will be displayed as disabled.
+/// (Like in the Material [Switch] widget.)
+///
 /// See Figma:
 ///    https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6070-60682&m=dev
 class Toggle extends StatelessWidget {
@@ -487,18 +490,29 @@ class Toggle extends StatelessWidget {
   });
 
   final bool value;
-  final ValueChanged<bool> onChanged;
+  final ValueChanged<bool>? onChanged;
 
   @override
   Widget build(BuildContext context) {
     // Figma has this (blue/500) in both light and dark mode.
     // TODO(#831)
     final activeColor = Color(0xff4370f0);
 
+    final activeColorDisabled = activeColor.withFadedAlpha(0.4);
+
     // Figma has this (grey/400) in both light and dark mode.
     // TODO(#831)
     final inactiveColor = Color(0xff9194a3);
 
+    final inactiveColorDisabled = inactiveColor.withFadedAlpha(0.4);
+
+    final trackColor = WidgetStateColor.fromMap({
+      WidgetState.selected  & ~WidgetState.disabled: activeColor,
+      WidgetState.selected  &  WidgetState.disabled: activeColorDisabled,
+      ~WidgetState.selected & ~WidgetState.disabled: inactiveColor,
+      ~WidgetState.selected &  WidgetState.disabled: inactiveColorDisabled,
+    });
+
     // TODO(#1636):
     //   All of these just need _SwitchConfig to be exposed,
     //   and there's an upstream issue for that:
@@ -524,8 +538,7 @@ class Toggle extends StatelessWidget {
       // Figma has white for "on" and "off" in both light and dark mode.
       thumbColor: WidgetStatePropertyAll(Colors.white),
 
-      activeTrackColor: activeColor,
-      inactiveTrackColor: inactiveColor,
+      trackColor: trackColor,
       trackOutlineColor: WidgetStatePropertyAll(Colors.transparent),
       trackOutlineWidth: WidgetStateProperty<double>.fromMap({
         // The outline is effectively painted with strokeAlignCenter: