feat: persist empty-state notice dismissal via user_meta

The "No active/completed notes found" notice was already dismissible
per-page-load (WP core's X button hides it), but pressing X only
cleared it for the current view — it returned on the next refresh.
The dismissal is now persisted to user_meta per-user-per-list-type,
so once closed it stays closed across page loads until the flag is
reset.

MECHANICS
- inc/wp-notes-display.php now checks get_user_meta(uid,
  'wp_notes_dismissed_empty_<type>') before rendering and skips the
  notice render entirely when set.
- New AJAX handler wp_ajax_wp_notes_dismiss_empty validates a nonce
  and the edit_posts capability, then writes the flag via
  update_user_meta(). type=active|completed; anything else 400s.
- An inline jQuery handler in wp_notes_add_inline_scripts() listens
  for clicks on .wp-notes-empty .notice-dismiss (WP core's auto-
  injected X button) and fires the AJAX call. The visual hide is
  still WP core's job.
- The notice element carries data-wp-notes-empty-type and a fresh
  per-render data-wp-notes-nonce for the round trip.

RESET
The flag is per-user-meta keyed wp_notes_dismissed_empty_active /
wp_notes_dismissed_empty_completed. wp_delete_user removes them
automatically. A UI button to reset dismissed notices is not built
yet — flagged in the changelog as a future enhancement.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

wp-notes.php

@@ -656,10 +656,47 @@ function wp_notes_add_inline_scripts() {
                     }
                 });
             });
+
+            // Persist the dismissal of the empty-state notice.
+            // WP core's common.js attaches the X button and hides the
+            // notice on click; we just hook the same click and fire an
+            // AJAX call to set the user_meta flag so the notice does
+            // not reappear on the next page load.
+            $(document).on('click', '.wp-notes-empty .notice-dismiss', function() {
+                var \$notice = $(this).closest('.wp-notes-empty');
+                var type   = \$notice.data('wp-notes-empty-type');
+                var nonce  = \$notice.data('wp-notes-nonce');
+                if (!type || !nonce) { return; }
+                $.post(ajaxurl, {
+                    action: 'wp_notes_dismiss_empty',
+                    type:   type,
+                    nonce:  nonce
+                });
+            });
         });
     </script>";
 }
 
+// Persist a user's dismissal of the "No active/completed notes found"
+// empty-state notice so it doesn't reappear on the next page load.
+// Triggered by inline JS in wp_notes_add_inline_scripts() when the
+// user clicks the X on a .wp-notes-empty notice.
+add_action('wp_ajax_wp_notes_dismiss_empty', 'wp_notes_ajax_dismiss_empty');
+function wp_notes_ajax_dismiss_empty() {
+    if (!current_user_can('edit_posts')) {
+        wp_send_json_error('Insufficient permissions.', 403);
+    }
+    check_ajax_referer('wp_notes_dismiss_empty', 'nonce');
+
+    $type = isset($_POST['type']) ? sanitize_key(wp_unslash($_POST['type'])) : '';
+    if (!in_array($type, ['active', 'completed'], true)) {
+        wp_send_json_error('Invalid list type.', 400);
+    }
+
+    update_user_meta(get_current_user_id(), 'wp_notes_dismissed_empty_' . $type, 1);
+    wp_send_json_success(['dismissed' => $type]);
+}
+
 // Handle saving edited notes
 add_action('wp_ajax_wp_notes_save_edit', 'wp_notes_save_edit');
 function wp_notes_save_edit() {