fix: use correct icons for chat pin/unpin

Similarly to the read/unread toggle this was also using the wrong icon.

If any selected room is not favourite, then show the pin icon (which
should represent the "mark as favourite" action as it corresponds to the
filled pin shown next to a favourited room. Otherwise show the outline
icon, representing the "unpin" action.

lib/pages/chat_list/chat_list_header.dart

@@ -147,8 +147,8 @@ class ChatListHeader extends StatelessWidget implements PreferredSizeWidget {
                     tooltip: L10n.of(context)!.toggleFavorite,
                     icon: Icon(
                       controller.anySelectedRoomNotFavorite
-                          ? Icons.push_pin_outlined
-                          : Icons.push_pin,
+                          ? Icons.push_pin
+                          : Icons.push_pin_outlined,
                     ),
                     onPressed: controller.toggleFavouriteRoom,
                   ),