better loading indicators for timetables, talk and files

lib/model/accountData.dart

@@ -98,8 +98,15 @@ class AccountData {
 
   bool isPopulated() => _username != null && _password != null;
 
-  String buildHttpAuthString() {
+  /// Returns the value for an HTTP `Authorization` header using HTTP Basic.
+  /// Prefer this over embedding credentials in URLs — error logs and crash
+  /// reports often capture the URL but not headers.
+  String getBasicAuthHeader() {
     if (!isPopulated()) throw Exception('AccountData (e.g. username or password) is not initialized!');
-    return '$_username:$_password';
+    return 'Basic ${base64Encode(utf8.encode('$_username:$_password'))}';
   }
+
+  /// Convenience wrapper around [getBasicAuthHeader] returning a single-entry
+  /// header map ready to merge into HTTP request headers.
+  Map<String, String> authHeaders() => {'Authorization': getBasicAuthHeader()};
 }