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/marianumconnect/queries/telemetry_heartbeat/telemetry_device_id.dart

@@ -1,5 +1,6 @@
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 
+import '../../../../storage/keychain_write.dart';
 import '../../../../utils/random_id.dart';
 
 /// A stable, anonymous per-install identifier for telemetry. Generated once on
@@ -22,7 +23,7 @@ class TelemetryDeviceId {
       return existing;
     }
     final generated = randomHexId();
-    await _storage.write(key: _key, value: generated);
+    await _storage.writeRetrying(key: _key, value: generated);
     _cached = generated;
     return generated;
   }