simplified code across the app and consolidated duplicated helpers

lib/push/push_avatar.dart

@@ -9,6 +9,7 @@ import 'package:http/http.dart' as http;
 import 'package:path_provider/path_provider.dart';
 
 import '../api/marianumcloud/nextcloud_ocs.dart';
+import 'chat_thread_store.dart';
 
 /// Result of an avatar lookup at render time: [icon] when bytes are available
 /// within the fetch budget, otherwise [late] carries the still-running fetch
@@ -102,13 +103,7 @@ class PushAvatarStore {
     },
   );
 
-  /// File-safe cache name for a chat token. Tokens are normally URL-safe
-  /// already and used verbatim; anything else is base64url-encoded so exotic
-  /// ids can never escape the cache directory.
-  static String fileNameForToken(String token) =>
-      RegExp(r'^[A-Za-z0-9_-]+$').hasMatch(token)
-      ? token
-      : base64Url.encode(utf8.encode(token)).replaceAll('=', '');
+  static String fileNameForToken(String token) => fileSafeToken(token);
 
   /// Whether a cache file written at [modified] is still fresh at [now].
   static bool isFresh(DateTime modified, DateTime now) =>