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/chat_thread_store.dart

@@ -1,5 +1,6 @@
 import 'dart:convert';
 
+import 'package:flutter/foundation.dart';
 import 'package:localstore/localstore.dart';
 
 /// Maximum messages kept per chat thread — enough for a messenger-style
@@ -190,8 +191,15 @@ class ChatThreadStore {
   DocumentRef _doc(String chatToken) =>
       _db.collection(_collection).doc(docIdForToken(chatToken));
 
-  Future<void> _write(String chatToken, List<ThreadMessage> messages) =>
-      _doc(chatToken).set({
+  /// Best effort: a full disk must not keep the notification from being
+  /// shown; the thread just restarts from the next push.
+  Future<void> _write(String chatToken, List<ThreadMessage> messages) async {
+    try {
+      await _doc(chatToken).set({
         'messages': [for (final m in messages) m.toJson()],
       });
+    } on Object catch (e) {
+      debugPrint('ChatThreadStore.write($chatToken) failed: $e');
+    }
+  }
 }