fix(busy): swallow the scrolling keys while a blocking operation runs

inert removes the controls from focus but not the document scroll: Space, PageUp/Down, Home/End and the arrows scroll without any focused element, so the page still moved under the overlay.

js/app.js

@@ -7410,6 +7410,18 @@ async function init() {
     // BEFORE any input-level handler that might call stopPropagation, and
     // before the browser swallows the keystroke for things like clearing
     // an active form-autocomplete popup on a focused field.
+    // `inert` takes the controls out of reach but does NOT stop the browser's
+    // own scrolling keys — Space, PageUp/Down, Home/End and the arrows scroll
+    // the document without needing a focused element. Swallow them while a
+    // blocking operation is running; everything else (Esc, shortcuts) is
+    // already handled by the guards on each feature.
+    document.addEventListener('keydown', e => {
+        if (!isBusy()) return;
+        const scrolls = [' ', 'Spacebar', 'PageUp', 'PageDown', 'Home', 'End',
+                         'ArrowUp', 'ArrowDown', 'ArrowLeft', 'ArrowRight'];
+        if (scrolls.indexOf(e.key) !== -1) e.preventDefault();
+    }, true);
+
     document.addEventListener('keydown', e => {
         if (e.key !== 'Escape') return;
         if (!$('#slideover').classList.contains('is-open')) return;