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/cache_store.dart

@@ -52,10 +52,16 @@ class CacheStore {
   Future<Directory> _directory() {
     final name = directoryName(SessionManager().activeAccount?.namespace);
     return _dirs[name] ??= () async {
-      final base = await getApplicationCacheDirectory();
-      final dir = Directory('${base.path}/$name');
-      await dir.create(recursive: true);
-      return dir;
+      try {
+        final base = await getApplicationCacheDirectory();
+        final dir = Directory('${base.path}/$name');
+        await dir.create(recursive: true);
+        return dir;
+      } on Object {
+        // Not memoized (disk full at launch): the next call tries again.
+        unawaited(_dirs.remove(name));
+        rethrow;
+      }
     }();
   }