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/widget/avatar_disk_cache.dart

@@ -39,18 +39,24 @@ class AvatarDiskCache {
 
   /// Kicks off directory resolution so [readSync] can hit on the very first
   /// avatar of a session. Fire-and-forget from app start; safe to call twice.
-  void warmUp() => unawaited(_directory());
+  void warmUp() => unawaited(_directory().then((_) {}, onError: (Object _) {}));
 
   Future<Directory> _directory() {
     return _dirFuture ??= _resolveDirectory();
   }
 
   Future<Directory> _resolveDirectory() async {
-    final base = await getApplicationCacheDirectory();
-    final dir = Directory('${base.path}/$_subDirectory');
-    await dir.create(recursive: true);
-    _dirPath = dir.path;
-    return dir;
+    try {
+      final base = await getApplicationCacheDirectory();
+      final dir = Directory('${base.path}/$_subDirectory');
+      await dir.create(recursive: true);
+      _dirPath = dir.path;
+      return dir;
+    } on Object {
+      // Not memoized (disk full at launch): the next call tries again.
+      _dirFuture = null;
+      rethrow;
+    }
   }
 
   /// File-safe, prefix-evictable name. The subject id is hex-encoded so it can