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

test/storage/keychain_write_test.dart

@@ -0,0 +1,35 @@
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:marianum_mobile/storage/keychain_write.dart';
+
+void main() {
+  group('isKeychainDuplicateItem', () {
+    test('recognises the OSStatus in the details', () {
+      final e = PlatformException(
+        code: 'Unexpected security result code',
+        message:
+            'Code: -25299, Message: The specified item already exists in the keychain.',
+        details: -25299,
+      );
+      expect(isKeychainDuplicateItem(e), isTrue);
+    });
+
+    test('falls back to the message when details are missing', () {
+      final e = PlatformException(
+        code: 'Unexpected security result code',
+        message: 'Code: -25299, Message: duplicate',
+      );
+      expect(isKeychainDuplicateItem(e), isTrue);
+    });
+
+    test('ignores other keychain errors and non-platform errors', () {
+      final e = PlatformException(
+        code: 'Unexpected security result code',
+        message: 'Code: -25308, Message: interaction not allowed',
+        details: -25308,
+      );
+      expect(isKeychainDuplicateItem(e), isFalse);
+      expect(isKeychainDuplicateItem(StateError('x')), isFalse);
+    });
+  });
+}