fix(hosting): disconnect the directory when update gets a null root_dir

puter.hosting.update(name, null) sends root_dir: null, which the driver
stringified into a lookup for a folder named "null" and rejected, so
disassociating a website from a folder's properties always failed.

src/backend/drivers/subdomain/SubdomainDriver.edges.test.ts

@@ -573,6 +573,24 @@ describe('SubdomainDriver.update field handling', () => {
         expect(cleared.associated_app).toBeNull();
     });
 
+    it('disconnects the directory when root_dir is null', async () => {
+        const { actor } = await makeUser();
+        const name = uniqueName('unlink');
+        await createSubdomain(actor, name);
+
+        // The shape `puter.hosting.update(name, null)` sends.
+        const updated = (await withActor(actor, () =>
+            driver.update({
+                id: { subdomain: name },
+                object: { root_dir: null },
+            }),
+        )) as Record<string, unknown>;
+
+        expect(updated.root_dir).toBeNull();
+        const row = await server.stores.subdomain.getBySubdomain(name);
+        expect(row?.root_dir_id ?? null).toBeNull();
+    });
+
     it('rejects repointing root_dir at a path that does not exist', async () => {
         const { actor } = await makeUser();
         const created = await createSubdomain(actor, uniqueName('badroot'));

src/backend/drivers/subdomain/SubdomainDriver.ts

@@ -357,7 +357,10 @@ export class SubdomainDriver extends PuterDriver {
 
         // Subdomain name is immutable — strip if provided
         const patch: Record<string, unknown> = {};
-        if (object.root_dir !== undefined) {
+        // A null root_dir disconnects the directory; the site stays reserved.
+        if (object.root_dir === null) {
+            patch.root_dir_id = null;
+        } else if (object.root_dir !== undefined) {
             const rootDirPath = expandTildePath(
                 String(object.root_dir),
                 actor.user.username,