Adds the bundled Folder Library plugin (browse the DLC library by its on-disk
folder tree, in-app folder CRUD, drag-and-drop + dialog song moves, sort/filter,
live search), wired into the classic v2 toolbar and the v3 Songs page.
Includes the screen.js IIFE dedup (unified surface factory) and review fixes:
path-traversal guard on /song/move, folder-delete data-loss fix, plural
/api/plugins/<id> namespace, loose-folder song recognition, error-text escaping,
v3 setLibView null-guard, and tests.
Co-authored-by: Kyle <kyle.j.t@live.co.uk>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
