fix: lock background scroll while a Modal or Sheet is open

Overlays didn't lock body scroll, so the page behind a modal/sheet scrolled
under it. Add a ref-counted scroll lock (so stacked overlays don't prematurely
release it) and hold it via an effect in Modal and Sheet while open, restoring
on close or unmount. Focus-trapping was already handled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.128.1",
+  "version": "0.128.2",
   "private": true,
   "type": "module",
   "scripts": {