fixed client-reported crashes in the pdf viewer, image picker and prosemirror images, mapped webdav errors to user messages and hardened keychain, cache and push stores against io failures

lib/api/errors/server_exception.dart

@@ -7,6 +7,7 @@ class ServerException extends AppException {
     required this.statusCode,
     String? userMessage,
     super.technicalDetails,
+    super.allowRetry,
   }) : super(
          // Status code intentionally not in the user message — it lives in
          // technicalDetails behind "Details anzeigen".
@@ -14,6 +15,5 @@ class ServerException extends AppException {
              userMessage ??
              'Der Server konnte die Anfrage gerade nicht verarbeiten. '
                  'Bitte versuche es in einem Moment erneut.',
-         allowRetry: true,
        );
 }