feat(pagination): keep the pager visible while scrolling

The page buttons sat at the top of the grid and scrolled away, so paging
through a long list meant scrolling back up. Sticky at the scrollport top
with an opaque background; the table header now sticks below it via
--pag-h (measured after append, so a wrapped pager still lines up).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/app.js

@@ -2297,7 +2297,12 @@ function renderGrid() {
     const pageList = list.slice(start, start + state.pageSize);
 
     if (total > 10) {
-        grid.appendChild(renderPagination(total, totalPages));
+        const pag = renderPagination(total, totalPages);
+        grid.appendChild(pag);
+        // The table header sticks below the sticky pagination bar, not under it.
+        grid.style.setProperty('--pag-h', pag.offsetHeight + 'px');
+    } else {
+        grid.style.setProperty('--pag-h', '0px');
     }
 
     if (state.viewMode === 'table') {