Fix modal-open layout shift caused by position:fixed scroll lock

Replace the body position:fixed hack with overflow:hidden on html element,
which works cleanly with scrollbar-gutter:stable to prevent layout shift.

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

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

@@ -91,10 +91,8 @@ body {
     line-height: 1.6;
 }
 
-body.modal-open {
-    position: fixed;
-    width: 100%;
-    overflow-y: scroll; /* keep scrollbar gutter to prevent layout shift */
+html.modal-open {
+    overflow: hidden; /* scrollbar-gutter: stable keeps the gutter reserved */
 }
 
 /* ── Ambient animated background ── */