List Files through get, because query cannot see a folder before 0.16

Creating a folder on the live 0.15.5 server did nothing visible, with no error
and nothing after a reload. The folder was real all along: FileNode/query masks
its results with document_ids(false) — resources that are *not* containers — so
it returns files and never folders, and says nothing about the omission.

FileNode/get carries no such mask, so on those servers the whole tree comes
from a single get with ids:null instead. That also stops ensureFolder making a
fresh "ihasmail" folder on every signature save, having never been able to find
the one already there.

web/src/lib/filenode.ts

@@ -24,6 +24,19 @@ export function supportsNodeType(): boolean {
 
 const BASE_PROPS = ["id", "parentId", "blobId", "size", "name", "type", "created", "modified", "myRights", "role", "executable"];
 
+/**
+ * Whether `FileNode/query` is blind to directories.
+ *
+ * Before 0.16 the query masks its results with `document_ids(false)`, which
+ * keeps only resources that are *not* containers — so it returns files and
+ * never folders, with no error to say so. A folder created there is real, and
+ * simply never comes back in a listing. `FileNode/get` has no such mask, so
+ * asking it for every id is the only way to see the whole tree.
+ */
+export function queryOmitsDirectories(): boolean {
+  return !supportsNodeType();
+}
+
 /** Properties to request, asking for `nodeType` only where it exists. */
 export function fileNodeProps(): string[] {
   return supportsNodeType() ? [...BASE_PROPS, "nodeType"] : BASE_PROPS;