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/storage/keychain_write.dart

@@ -0,0 +1,34 @@
+import 'package:flutter/services.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+/// `errSecDuplicateItem`: the iOS keychain refused to add an item that is
+/// already there.
+const int kErrSecDuplicateItem = -25299;
+
+/// Whether [error] is the iOS keychain's "item already exists" failure.
+/// flutter_secure_storage surfaces it as a PlatformException whose details
+/// (and message) carry the OSStatus.
+bool isKeychainDuplicateItem(Object error) {
+  if (error is! PlatformException) return false;
+  if (error.details == kErrSecDuplicateItem) return true;
+  return error.message?.contains('$kErrSecDuplicateItem') ?? false;
+}
+
+extension KeychainWrite on FlutterSecureStorage {
+  /// [write] that survives `errSecDuplicateItem`. The plugin's iOS write is
+  /// check-then-add/update and not atomic: a concurrent write to the same key
+  /// — or an existence check that itself failed (seen with the disk full) —
+  /// ends in SecItemAdd on an existing item. The second attempt finds the
+  /// item and updates it.
+  Future<void> writeRetrying({
+    required String key,
+    required String value,
+  }) async {
+    try {
+      await write(key: key, value: value);
+    } on PlatformException catch (e) {
+      if (!isKeychainDuplicateItem(e)) rethrow;
+      await write(key: key, value: value);
+    }
+  }
+}