fix(documents): folderId=empty-string normalises to null at validator

The hub UI sends folderId='' when the user picks "root-only" in the
folder sidebar. The Zod validator was accepting it as a string and
the service then ran eq(folderId, '') instead of isNull(folderId),
returning zero results.

Adding a .transform on folderId converts empty string to null at the
boundary so the service receives the expected nullable shape. Pre-
existing bug from Wave 11.B that the hub rebuild made more visible.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/validators/documents.ts

@@ -86,7 +86,11 @@ export const listDocumentsSchema = baseListQuerySchema
     interestId: z.string().optional(),
     clientId: z.string().optional(),
     documentType: z.string().optional(),
-    folderId: z.string().nullable().optional(),
+    folderId: z
+      .string()
+      .nullable()
+      .optional()
+      .transform((v) => (v === '' ? null : v)),
     includeDescendants: z.coerce.boolean().optional(),
     status: z.string().optional(),
     /** Hub tab filter - applies tab-specific status / signer-membership constraints. */