compose: Replace compose box with error banner for archived channels

Author: chimnayajith

lib/widgets/compose_box.dart

@@ -1423,8 +1423,10 @@ class _ComposeBoxState extends State<ComposeBox> with PerAccountStoreAwareStateM
       case ChannelNarrow(:final streamId):
       case TopicNarrow(:final streamId):
         final channel = store.streams[streamId];
-        if (channel == null || !store.hasPostingPermission(inChannel: channel,
-            user: store.selfUser, byDate: DateTime.now())) {
+       if (channel == null
+            || !store.hasPostingPermission(inChannel: channel,
+                 user: store.selfUser, byDate: DateTime.now())
+            || channel.isArchived) {
           return _ErrorBanner(label:
             ZulipLocalizations.of(context).errorBannerCannotPostInChannelLabel);
         }

test/widgets/compose_box_test.dart

@@ -951,6 +951,34 @@ void main() {
       });
     });
 
+    group('in channel/topic narrow with archived channels', () {
+      void checkComposeBox({required bool isShown}) => checkComposeBoxIsShown(isShown,
+        bannerLabel: zulipLocalizations.errorBannerCannotPostInChannelLabel);
+
+      final narrowTestCases = [
+        ('channel', const ChannelNarrow(1)),
+        ('topic',   eg.topicNarrow(1, 'topic')),
+      ];
+
+      for (final (String narrowType, Narrow narrow) in narrowTestCases) {
+        testWidgets('error banner is shown in $narrowType narrow', (tester) async {
+          await prepareComposeBox(tester,
+            narrow: narrow,
+            selfUser: eg.user(),
+            streams: [eg.stream(streamId: 1, isArchived: true)]);
+          checkComposeBox(isShown: false);
+        });
+
+        testWidgets('compose box is shown in $narrowType narrow', (tester) async {
+          await prepareComposeBox(tester,
+            narrow: narrow,
+            selfUser: eg.user(role: UserRole.administrator),
+            streams: [eg.stream(streamId: 1, isArchived: false)]);
+          checkComposeBox(isShown: true);
+        });
+      }
+    });
+
     group('in channel/topic narrow according to channel post policy', () {
       void checkComposeBox({required bool isShown}) => checkComposeBoxIsShown(isShown,
         bannerLabel: zulipLocalizations.errorBannerCannotPostInChannelLabel);