fix: standardize Old Book → Old Books folder name

- filebrowserFolders.js: createTaskFolder now creates Old Books
- fbq-backfill: creates Old Books; migrateOldBook() merges/renames
  existing Old Book folders into Old Books on each backfill run

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/filebrowserFolders.js

@@ -50,7 +50,7 @@ export async function createTaskFolder(companyName, projectName, taskTitle) {
     `/Clients/${co}/Projects/${proj}/00 Project Files/`,
     `${base}/`,
     `${base}/Request Info/`,
-    `${base}/Old Book/`,
+    `${base}/Old Books/`,
   ];
   for (const dir of mkdirs) {
     await fbq('mkdir', dir);