Refactor admin modals: replace backdrop divs with native <dialog> elements for improved accessibility and functionality

admin/index.js

@@ -42,3 +42,14 @@ document.addEventListener('submit', (e) => {
     e.preventDefault();
   }
 });
+
+// Announcement editor dialog: the server flags it for ?new / ?edit URLs.
+// Closing it any way (× link, Esc) drops the query string, so a reload
+// doesn't reopen it.
+const annModal = document.getElementById('modal-ann');
+if (annModal?.hasAttribute('data-open-on-load')) {
+  annModal.showModal();
+  annModal.addEventListener('close', () => {
+    history.replaceState(null, '', 'index.php');
+  });
+}