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/push/push_secure_storage.dart

@@ -1,4 +1,5 @@
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import '../storage/keychain_write.dart';
 
 /// Keychain access group shared between the Runner and the (Phase 3) iOS
 /// Notification Service Extension so the NSE can read the RSA private key,
@@ -48,7 +49,9 @@ class PushSecureStorage implements FlutterSecureStorageLike {
 
   @override
   Future<void> write({required String key, required String? value}) =>
-      pushSecureStorage.write(key: key, value: value);
+      value == null
+      ? pushSecureStorage.delete(key: key)
+      : pushSecureStorage.writeRetrying(key: key, value: value);
 
   @override
   Future<void> delete({required String key}) =>