app_bar [nfc]: Explain why the lightbox skips ZulipAppBar

Just listing that there is an exception doesn't help much to provide
guidance for someone writing a new page on whether they should also
skip ZulipAppBar there.  The "why" is much more helpful for that.

Right at the call site is also more helpful than at the definition
of ZulipAppBar, because the call site is the place someone will
definitely be looking if they're contemplating whether this
deviation from the pattern is appropriate, a mistake, or just an
oversight.

Author: gnprice

lib/widgets/app_bar.dart

@@ -5,8 +5,6 @@ import 'store.dart';
 /// A custom [AppBar] with a loading indicator.
 ///
 /// This should be used for most of the pages with access to [PerAccountStore].
-// However, there are some exceptions (add more if necessary):
-// - `lib/widgets/lightbox.dart`
 class ZulipAppBar extends AppBar {
   ZulipAppBar({
     super.key,

lib/widgets/lightbox.dart

@@ -157,6 +157,13 @@ class _LightboxPageLayoutState extends State<_LightboxPageLayout> {
         .add_Hms()
         .format(DateTime.fromMillisecondsSinceEpoch(widget.message.timestamp * 1000));
 
+      // We use plain [AppBar] instead of [ZulipAppBar], even though this page
+      // has a [PerAccountStore], because:
+      //  * There's already a progress indicator with a different meaning
+      //    (loading the image).
+      //  * The app bar can be hidden, so wouldn't always be visible anyway.
+      //  * This is a page where the store loading indicator isn't especially
+      //    necessary: https://github.com/zulip/zulip-flutter/pull/852#issuecomment-2264211917
       appBar = AppBar(
         centerTitle: false,
         foregroundColor: appBarForegroundColor,