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 ── */

server/src/wled_controller/static/js/core/modal.js

@@ -43,7 +43,7 @@ export class Modal {
         this.onForceClose();
         Modal._stack = Modal._stack.filter(m => m !== this);
         if (this._previousFocus && typeof this._previousFocus.focus === 'function') {
-            this._previousFocus.focus();
+            this._previousFocus.focus({ preventScroll: true });
             this._previousFocus = null;
         }
     }

server/src/wled_controller/static/js/core/ui.js

@@ -84,7 +84,7 @@ export function unlockBody() {
     if (_lockCount === 0) {
         document.body.classList.remove('modal-open');
         document.body.style.top = '';
-        window.scrollTo(0, _savedScrollY);
+        window.scrollTo({ top: _savedScrollY, behavior: 'instant' });
     }
 }