Fix scroll position reset when closing modals

- Use { behavior: 'instant' } in unlockBody scrollTo to override
  CSS scroll-behavior: smooth on html element
- Use { preventScroll: true } on focus() restore in Modal.forceClose
- Add overflow-y: scroll to body.modal-open to prevent scrollbar shift

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/wled_controller/static/css/base.css

@@ -93,6 +93,7 @@ body {
 body.modal-open {
     position: fixed;
     width: 100%;
+    overflow-y: scroll; /* keep scrollbar gutter to prevent layout shift */
 }
 
 /* ── Ambient animated background ── */