style: verbleibende flutter-analyze Deprecations beheben

withOpacity/activeColor/cacheExtent/PublicRoomsChunk durch die neuen
Flutter-3.44-Äquivalente ersetzt.

lib/features/chat/chat_view.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
+import 'package:flutter/rendering.dart' show ScrollCacheExtent;
 import 'package:flutter/services.dart' show Clipboard, ClipboardData;
 
 import 'package:desktop_drop/desktop_drop.dart';
@@ -1598,7 +1599,7 @@ class _MessageList extends StatelessWidget {
       reverse: true,
       physics: const ClampingScrollPhysics(),
       padding: const EdgeInsets.only(bottom: 8),
-      cacheExtent: 2500,
+      scrollCacheExtent: const ScrollCacheExtent.pixels(2500),
       itemCount: itemKeys.length,
       findChildIndexCallback: (Key key) {
         // Only return an index when the key still maps to that exact slot —