refactored room and file sorting to use direct comparators instead of temporary sort strings, removed obsolete 'sort' properties from API models, and improved file list sorting with case-insensitive name comparisons and null-safe date handling

lib/view/pages/files/data/sort_options.dart

@@ -21,12 +21,16 @@ class SortOptions {
     SortOption.name: BetterSortOption(
       displayName: 'Name',
       icon: Icons.sort_by_alpha_outlined,
-      compare: (a, b) => a.name.compareTo(b.name),
+      compare: (a, b) => a.name.toLowerCase().compareTo(b.name.toLowerCase()),
     ),
     SortOption.date: BetterSortOption(
       displayName: 'Datum',
       icon: Icons.history_outlined,
-      compare: (a, b) => a.modifiedAt!.compareTo(b.modifiedAt!),
+      compare: (a, b) {
+        final aMs = a.modifiedAt?.millisecondsSinceEpoch ?? 0;
+        final bMs = b.modifiedAt?.millisecondsSinceEpoch ?? 0;
+        return aMs.compareTo(bMs);
+      },
     ),
     SortOption.size: BetterSortOption(
       displayName: 'Größe',

lib/view/pages/share_intent/share_chat_picker.dart

@@ -100,7 +100,6 @@ class ShareChatPicker extends StatelessWidget {
           if (rooms == null) return const SizedBox.shrink();
           final sorted = rooms
               .sortBy(
-                lastActivity: true,
                 favoritesToTop: talkSettings.sortFavoritesToTop,
                 unreadToTop: talkSettings.sortUnreadToTop,
               )

lib/view/pages/talk/chat_list.dart

@@ -162,7 +162,6 @@ class _ChatListViewState extends State<_ChatListView> {
                   .val()
                   .talkSettings;
               final sorted = rooms.sortBy(
-                lastActivity: true,
                 favoritesToTop: talkSettings.sortFavoritesToTop,
                 unreadToTop: talkSettings.sortUnreadToTop,
               );