msglist: Remove extra recipient header when topic changes case

This change makes the message list's topic headers case-insensitive
Messages with topics differing only in case (e.g., "missing string"
and "Missing string") will now share a single header. This aligns
the behavior with Zulip web and ensures a consistent user experience.

Fixes: #739

lib/model/message_list.dart

@@ -321,7 +321,7 @@ mixin _MessageSequence {
 bool haveSameRecipient(Message prevMessage, Message message) {
   if (prevMessage is StreamMessage && message is StreamMessage) {
     if (prevMessage.streamId != message.streamId) return false;
-    if (prevMessage.topic != message.topic) return false;
+    if (prevMessage.topic.toLowerCase() != message.topic.toLowerCase()) return false;
   } else if (prevMessage is DmMessage && message is DmMessage) {
     if (!_equalIdSequences(prevMessage.allRecipientIds, message.allRecipientIds)) {
       return false;

test/model/message_list_test.dart

@@ -1704,6 +1704,25 @@ void main() {
       }
     });
   });
+  group('topics compared case-insensitively', () {
+    void doTest(String description, String topicA, String topicB, bool expected) {
+      test(description, () {
+        final stream = eg.stream();
+        final messageA = eg.streamMessage(stream: stream, topic: topicA);
+        final messageB = eg.streamMessage(stream: stream, topic: topicB);
+        check(haveSameRecipient(messageA, messageB)).equals(expected);
+      });
+    }
+
+    doTest('same case, all lower',               'abc',  'abc',  true);
+    doTest('same case, all upper',               'ABC',  'ABC',  true);
+    doTest('same case, mixed',                   'AbC',  'AbC',  true);
+    doTest('same non-letter chars',              '嗎',    '嗎',    true);
+    doTest('different case',                     'aBc',  'ABC',  true);
+    doTest('different case, same diacritics',    'AbÇ',  'aBç',  true);
+    doTest('same letters, different diacritics', 'ma',   'mǎ',   false);
+    doTest('having different non-letter chars',  'ma 嗎', 'mǎ 馬', false);
+  });
 
   test('messagesSameDay', () {
     // These timestamps will differ depending on the timezone of the