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/__tests__/filenode.test.ts

@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, it } from "vitest";
 import { client } from "@/jmap/client";
-import { directoryCreate, fileCreate, fileNodeProps, supportsNodeType, normalizeFileNodes } from "../filenode";
+import { directoryCreate, fileCreate, fileNodeProps, normalizeFileNodes, queryOmitsDirectories, supportsNodeType } from "../filenode";
 import type { FileNode, JmapSession } from "@/jmap/types";
 
 /**
@@ -117,3 +117,21 @@ describe("rights on a pre-0.16 server", () => {
     expect(node!.nodeType).toBe("directory");
   });
 });
+
+/**
+ * Before 0.16, FileNode/query masks its results with `document_ids(false)` —
+ * only resources that are *not* containers. It therefore returns files and
+ * never folders, with no error to explain the omission: a folder created there
+ * exists but never comes back in a listing. FileNode/get carries no such mask.
+ */
+describe("directory-blind query", () => {
+  it("is worked around on older servers", () => {
+    client.session = session(OLD_SERVER);
+    expect(queryOmitsDirectories()).toBe(true);
+  });
+
+  it("is not worked around where query can see folders", () => {
+    client.session = session(NEW_SERVER);
+    expect(queryOmitsDirectories()).toBe(false);
+  });
+});