fix(chat): clear unread state for messages read in an open chat

Messages arriving while a conversation was open were never marked as read,
so the chat list kept showing them as unread after going back. The screen
now marks incoming messages read while it is on top and the app is in the
foreground, again when the app returns to the foreground, and on leaving.

Read state is now monotonic in Conversation.updateMessage, so a stale copy
of a message (attachment download, delayed resend) can no longer flip it
back to unread.

Also create the swipe-to-reply animation controller lazily instead of in
dispose().

app/test/conversation_read_flag_test.dart

@@ -0,0 +1,37 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sepot/models/conversation.dart';
+import 'package:sepot/models/message.dart';
+
+/// Read state only moves forward. Updates that carry an older copy of the
+/// message (attachment download finishing, delivery status) must not flip a
+/// message the user has already read back to unread.
+void main() {
+  Message zprava({bool isRead = false}) => Message(
+    id: 'm1',
+    senderId: 'alice',
+    recipientId: 'bob',
+    type: MessageType.text,
+    content: 'hi',
+    timestamp: DateTime.parse('2026-10-10T09:00:00.000'),
+    isRead: isRead,
+  );
+
+  test('stale unread copy does not undo a read message', () {
+    final c = Conversation(contactId: 'alice')..addMessage(zprava());
+    c.markAllRead('bob');
+
+    c.updateMessage(zprava().copyWith(content: 'hi (edited)'));
+
+    expect(c.messages.single.isRead, isTrue);
+    expect(c.messages.single.content, 'hi (edited)');
+    expect(c.unreadCount('bob'), 0);
+  });
+
+  test('update can still mark an unread message as read', () {
+    final c = Conversation(contactId: 'alice')..addMessage(zprava());
+
+    c.updateMessage(zprava(isRead: true));
+
+    expect(c.unreadCount('bob'), 0);
+  });
+}